cmd/compile: do not skip TestFormats on short builders

TestFormats adds ~3s of running time to the test, which may be
slightly annoying in an edit/compile/test cycle but is negligible in a
TryBot run.

The test keeps regressing in the longtest builders, requiring a manual
fix. Instead, run it even in short mode on the builders, so that
TryBot runs will detect regressions ahead of time.

Updates #34907
Updates #33915
Updates #28621

Change-Id: I6f9bf0f2ca929a743438310b86d85d8673c720bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/205440
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/fmt_test.go

@@ -83,7 +83,7 @@ type File struct {
 }
 
 func TestFormats(t *testing.T) {
-	if testing.Short() {
+	if testing.Short() && testenv.Builder() == "" {
 		t.Skip("Skipping in short mode")
 	}
 	testenv.MustHaveGoBuild(t) // more restrictive than necessary, but that's ok