os, os/exec, os/user: add support for GOOS=solaris

R=golang-codereviews, dave, minux.ma, gobot, jsing
CC=golang-codereviews
https://golang.org/cl/36020043
diff --git a/src/pkg/os/os_unix_test.go b/src/pkg/os/os_unix_test.go
index 1e8a661..21d40cc 100644
--- a/src/pkg/os/os_unix_test.go
+++ b/src/pkg/os/os_unix_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package os_test