compress/gzip: skip mtime test when GOROOT doesn't exist

Fixes the iOS builders

Change-Id: I5097ca19048381ffb5a4c5ea038b7c4aa18ee4b7
Reviewed-on: https://go-review.googlesource.com/21132Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/issue14937_test.go

@@ -23,6 +23,9 @@ func TestGZIPFilesHaveZeroMTimes(t *testing.T) {
 		return nil
 	})
 	if err != nil {
+		if os.IsNotExist(err) {
+			t.Skipf("skipping: GOROOT directory not found: %s", runtime.GOROOT())
+		}
 		t.Fatal("error collecting list of .gz files in GOROOT: ", err)
 	}
 	if len(files) == 0 {