runtime: skip TestRuntimeLockMetricsAndProfile for flakiness

This test was added to check new mutex profile functionality.
Specifically, it checks to make sure that the functionality behind
GODEBUG=runtimecontentionstacks works. The runtime currently tracks
contention from runtime-internal mutexes in mutex profiles, but it does
not record stack traces for them, attributing the time to a dummy
symbol. This GODEBUG enables collecting stacks.

Just disable the test. Even if this functionality breaks, it won't
affect Go users and it'll help keep the builders green. It's fine to
leave the test because this will be revisited in the next dev cycle.

For #64253.

Change-Id: I7938fe0f036fc4e4a0764f030e691e312ec2c9b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/550775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/runtime/metrics_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"internal/goexperiment"
 	"internal/profile"
+	"internal/testenv"
 	"os"
 	"reflect"
 	"runtime"
@@ -947,6 +948,8 @@ func TestSchedPauseMetrics(t *testing.T) {
 }
 
 func TestRuntimeLockMetricsAndProfile(t *testing.T) {
+	testenv.SkipFlaky(t, 64253)
+
 	old := runtime.SetMutexProfileFraction(0) // enabled during sub-tests
 	defer runtime.SetMutexProfileFraction(old)
 	if old != 0 {