cmd/gofmt: don't descend into /testdata directories in tests (fix long builders)

Now that the go/types and types2 test files end in .go we must
avoid trying to format them as that won't work in general.

Change-Id: I05fdd95a0d26cbe746f6d618b22b48dc1f1ea749
Reviewed-on: https://go-review.googlesource.com/c/go/+/394295
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/gofmt/long_test.go

@@ -114,7 +114,8 @@ func genFilenames(t *testing.T, filenames chan<- string) {
 			t.Error(err)
 			return nil
 		}
-		if isGoFile(d) {
+		// don't descend into testdata directories
+		if isGoFile(d) && !strings.Contains(filepath.ToSlash(filename), "/testdata/") {
 			filenames <- filename
 			nfiles++
 		}