context: disable more flaky tests on openbsd

Updates #15158

Change-Id: Icb3788152a7a5a9b0d56ea38da46d770ffdce413
Reviewed-on: https://go-review.googlesource.com/21763
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/context/context_test.go

@@ -494,6 +494,9 @@ func TestLayersTimeout(t *testing.T) {
 }
 
 func testLayers(t *testing.T, seed int64, testTimeout bool) {
+	if runtime.GOOS == "openbsd" {
+		testenv.SkipFlaky(t, 15158)
+	}
 	rand.Seed(seed)
 	errorf := func(format string, a ...interface{}) {
 		t.Errorf(fmt.Sprintf("seed=%d: %s", seed, format), a...)