cmd/go: skip TestScript on Plan 9 in short mode

TestScript is very slow on Plan 9 because this test
is particularly i/o intensive.

This is leading the plan9/386 and plan9/amd64 builders
to time out. This test was already skipped on plan9/arm
because arm is part of the "slow architectures" list.

This change skips TestScript on Plan 9 on short mode.

Change-Id: I3e68046dac825cd14fa8daca601c492cf11c6fff
Reviewed-on: https://go-review.googlesource.com/c/go/+/614855
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Ian Lance Taylor <iant@golang.org>

src/cmd/go/script_test.go

@@ -41,6 +41,10 @@ func TestScript(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	testenv.SkipIfShortAndSlow(t)
 
+	if testing.Short() && runtime.GOOS == "plan9" {
+		t.Skipf("skipping test in -short mode on %s", runtime.GOOS)
+	}
+
 	srv, err := vcstest.NewServer()
 	if err != nil {
 		t.Fatal(err)