Fix incorrect assert in AbandonExisting test case (#55198)

* Handle the case where the "notAbandonedWait" event is not signaled

Author: uweigand

src/libraries/System.Threading/tests/MutexTests.cs

@@ -348,7 +348,7 @@ public void AbandonExisting(
                             }
                             else
                             {
-                                Assert.True(m2Index < notAbandonedWaitIndex);
+                                Assert.True(!isNotAbandonedWaitObjectSignaled || m2Index < notAbandonedWaitIndex);
                                 Assert.Equal(m2Index, ame.MutexIndex);
                             }