runtime: fix test of when a mutex is contended

This is used only in tests that verify reports of runtime-internal mutex
contention.

For golang#66999
For golang#70602

Change-Id: I72cb1302d8ea0524f1182ec892f5c9a1923cddba
Reviewed-on: https://go-review.googlesource.com/c/go/+/667095
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

Author: rhysh

src/runtime/lock_spinbit.go

@@ -143,7 +143,7 @@ func mutexPreferLowLatency(l *mutex) bool {
 }
 
 func mutexContended(l *mutex) bool {
-	return atomic.Loaduintptr(&l.key) > mutexLocked
+	return atomic.Loaduintptr(&l.key)&^mutexMMask != 0
 }
 
 func lock(l *mutex) {