cmd/compile: run fmt test only in long mode

Update #26469

Change-Id: Id8b8d1c0db48374d5d3dc663a77187a73f60c9a5
Reviewed-on: https://go-review.googlesource.com/125037
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/fmt_test.go

@@ -73,6 +73,9 @@ type File struct {
 }
 
 func TestFormats(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
 	testenv.MustHaveGoBuild(t) // more restrictive than necessary, but that's ok
 
 	// process all directories