testing: fix racey access to t.failed

We need to grab the mutex before we can access it.

Fixes #24438

Change-Id: Idd6130036691acec5bc5f8b40d6884f8db1d9d3c
Reviewed-on: https://go-review.googlesource.com/101283
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -731,7 +731,7 @@ func tRunner(t *T, fn func(t *T)) {
 	// a call to runtime.Goexit, record the duration and send
 	// a signal saying that the test is done.
 	defer func() {
-		if t.failed {
+		if t.Failed() {
 			atomic.AddUint32(&numFailed, 1)
 		}