compress/gzip: skip builder-only test on iOS

The new iOS test harness forwards environment variables to the
test program, which means that it runs builder-only tests that were
previously skipped because GO_BUILDER_NAME was missing.

Skip one such unblocked test, TestGZIPFilesHaveZeroMTimes, which
assumes a valid GOROOT.

Change-Id: I5daf0f4c1897afbeb8b1a380669a1d2aa47e764a
Reviewed-on: https://go-review.googlesource.com/111475
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/issue14937_test.go

@@ -21,6 +21,10 @@ func TestGZIPFilesHaveZeroMTimes(t *testing.T) {
 	if testenv.Builder() == "" {
 		t.Skip("skipping test on non-builder")
 	}
+	if !testenv.HasSrc() {
+		t.Skip("skipping; no GOROOT available")
+	}
+
 	goroot, err := filepath.EvalSymlinks(runtime.GOROOT())
 	if err != nil {
 		t.Fatal("error evaluating GOROOT: ", err)