runtime/pprof: mark TestMutexProfile as flaky for now

Flaky tests hurt productivity. Disable for now.

Updates #19139

Change-Id: I2e3040bdf0e53597a1c4f925b788e3268ea284c1
Reviewed-on: https://go-review.googlesource.com/37291
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/pprof/pprof_test.go

@@ -511,6 +511,7 @@ func blockCond() {
 }
 
 func TestMutexProfile(t *testing.T) {
+	testenv.SkipFlaky(t, 19139)
 	old := runtime.SetMutexProfileFraction(1)
 	defer runtime.SetMutexProfileFraction(old)
 	if old != 0 {