context: mark testDeadline as a testing helper method

Change-Id: Ie6fc3e9789aea6e5949e66186db6f2b071b6fdff
Reviewed-on: https://go-review.googlesource.com/c/go/+/204037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/context_test.go

@@ -253,6 +253,7 @@ func XTestChildFinishesFirst(t testingT) {
 }
 
 func testDeadline(c Context, name string, failAfter time.Duration, t testingT) {
+	t.Helper()
 	select {
 	case <-time.After(failAfter):
 		t.Fatalf("%s: context should have timed out", name)