all: make tests able to run multiple times.

It is now possible to run "go test -cpu=1,2,4 std"
successfully.

Fixes #3185.

R=golang-dev, dave, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/7196052
diff --git a/src/pkg/expvar/expvar_test.go b/src/pkg/expvar/expvar_test.go
index bbd9dd8..572c62b 100644
--- a/src/pkg/expvar/expvar_test.go
+++ b/src/pkg/expvar/expvar_test.go
@@ -18,6 +18,7 @@
 }
 
 func TestInt(t *testing.T) {
+	RemoveAll()
 	reqs := NewInt("requests")
 	if reqs.i != 0 {
 		t.Errorf("reqs.i = %v, want 0", reqs.i)
@@ -43,6 +44,7 @@
 }
 
 func TestFloat(t *testing.T) {
+	RemoveAll()
 	reqs := NewFloat("requests-float")
 	if reqs.f != 0.0 {
 		t.Errorf("reqs.f = %v, want 0", reqs.f)
@@ -68,6 +70,7 @@
 }
 
 func TestString(t *testing.T) {
+	RemoveAll()
 	name := NewString("my-name")
 	if name.s != "" {
 		t.Errorf("name.s = %q, want \"\"", name.s)
@@ -84,6 +87,7 @@
 }
 
 func TestMapCounter(t *testing.T) {
+	RemoveAll()
 	colors := NewMap("bike-shed-colors")
 
 	colors.Add("red", 1)
@@ -123,6 +127,7 @@
 }
 
 func TestFunc(t *testing.T) {
+	RemoveAll()
 	var x interface{} = []string{"a", "b"}
 	f := Func(func() interface{} { return x })
 	if s, exp := f.String(), `["a","b"]`; s != exp {