runtime/pprof: plan9 fails the TestGoroutineSwitch, skip for now.

LGTM=r
R=golang-codereviews, 0intro, r
CC=golang-codereviews
https://golang.org/cl/55430043

src/pkg/runtime/pprof/pprof_test.go

@@ -267,6 +267,7 @@ func TestMathBigDivide(t *testing.T) {
 var badOS = map[string]bool{
 	"darwin": true,
 	"netbsd": true,
+	"plan9":  true,
 }
 
 func TestBlockProfile(t *testing.T) {