_content/tour: fix content test

- Add missing '// +build' comments for generics examples.
- Add build tags 'OMIT', 'no-run' for solutions examples.
- Move content_test.go to the root because 'go' ignores
packages in the '_content' directory.
- Remove usages of deprecated 'io/ioutil'.

Change-Id: I8e6b717d43df9e2b55e4b796db5b316860f42103
Reviewed-on: https://go-review.googlesource.com/c/website/+/592875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/_content/tour/solutions/binarytrees.go b/_content/tour/solutions/binarytrees.go
index 2ba3604..b16b960 100644
--- a/_content/tour/solutions/binarytrees.go
+++ b/_content/tour/solutions/binarytrees.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/binarytrees_quit.go b/_content/tour/solutions/binarytrees_quit.go
index 076bd66..0bdc20b 100644
--- a/_content/tour/solutions/binarytrees_quit.go
+++ b/_content/tour/solutions/binarytrees_quit.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2015 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/errors.go b/_content/tour/solutions/errors.go
index e8de49e..3b10238 100644
--- a/_content/tour/solutions/errors.go
+++ b/_content/tour/solutions/errors.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/fib.go b/_content/tour/solutions/fib.go
index 0f3959a..1a462ac 100644
--- a/_content/tour/solutions/fib.go
+++ b/_content/tour/solutions/fib.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import "fmt"
diff --git a/_content/tour/solutions/http.go b/_content/tour/solutions/http.go
index 9fe3d5f..1ffa327 100644
--- a/_content/tour/solutions/http.go
+++ b/_content/tour/solutions/http.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore no-run
+
 // Copyright 2014 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/image.go b/_content/tour/solutions/image.go
index 60d9c5e..38a6b77 100644
--- a/_content/tour/solutions/image.go
+++ b/_content/tour/solutions/image.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/loops.go b/_content/tour/solutions/loops.go
index 296138f..06fe43f 100644
--- a/_content/tour/solutions/loops.go
+++ b/_content/tour/solutions/loops.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/maps.go b/_content/tour/solutions/maps.go
index ddd6b52..6a1eee0 100644
--- a/_content/tour/solutions/maps.go
+++ b/_content/tour/solutions/maps.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/readers.go b/_content/tour/solutions/readers.go
index 278cd65..82694c5 100644
--- a/_content/tour/solutions/readers.go
+++ b/_content/tour/solutions/readers.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2017 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import "golang.org/x/tour/reader"
diff --git a/_content/tour/solutions/rot13.go b/_content/tour/solutions/rot13.go
index 84315a4..8edce64 100644
--- a/_content/tour/solutions/rot13.go
+++ b/_content/tour/solutions/rot13.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (
diff --git a/_content/tour/solutions/slices.go b/_content/tour/solutions/slices.go
index 37bf9b3..0237557 100644
--- a/_content/tour/solutions/slices.go
+++ b/_content/tour/solutions/slices.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import "golang.org/x/tour/pic"
diff --git a/_content/tour/solutions/stringers.go b/_content/tour/solutions/stringers.go
index 7462f4c..98c26d1 100644
--- a/_content/tour/solutions/stringers.go
+++ b/_content/tour/solutions/stringers.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2015 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import "fmt"
diff --git a/_content/tour/solutions/webcrawler.go b/_content/tour/solutions/webcrawler.go
index 89b86a8..332cf6a 100644
--- a/_content/tour/solutions/webcrawler.go
+++ b/_content/tour/solutions/webcrawler.go
@@ -1,9 +1,9 @@
+// +build OMIT ignore
+
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package main
 
 import (