testing: fix Cleanup race with Logf and Errorf

Fixes #40908

Change-Id: I25561a3f18e730a50e6fbf85aa7bd85bf1b73b6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/250078
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/testing/testing.go

@@ -860,11 +860,15 @@ func (c *common) Cleanup(f func()) {
 	c.cleanup = func() {
 		if oldCleanup != nil {
 			defer func() {
+				c.mu.Lock()
 				c.cleanupPc = oldCleanupPc
+				c.mu.Unlock()
 				oldCleanup()
 			}()
 		}
+		c.mu.Lock()
 		c.cleanupName = callerName(0)
+		c.mu.Unlock()
 		f()
 	}
 	var pc [maxStackLen]uintptr