[KMTESTS:EX] Try to avoid some spurious test failures.

E.g. https://reactos.org/testman/detail.php?id=43906459&prev=43889679

This test depends on the timeout to be long enough for another thread to
actually have started waiting, which may take longer than 10ms under load.

modules/rostests/kmtests/ntos_ex/ExFastMutex.c

@@ -253,7 +253,7 @@ TestFastMutexConcurrent(
     THREAD_DATA ThreadDataUnsafe;
     THREAD_DATA ThreadDataTry;
     LARGE_INTEGER Timeout;
-    Timeout.QuadPart = -10 * 1000 * 10; /* 10 ms */
+    Timeout.QuadPart = -50 * MILLISECOND;
 
     InitThreadData(&ThreadData, Mutex, ExAcquireFastMutex, NULL, ExReleaseFastMutex);
     InitThreadData(&ThreadData2, Mutex, ExAcquireFastMutex, NULL, ExReleaseFastMutex);