tst_QThreadPool: remove Windows-specific QSKIP

I cannot get it to fail on my local machine

Change-Id: Iec30858df6bf5ef51a805745745cc0e98e8db03a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp

@@ -418,9 +418,6 @@ void tst_QThreadPool::expiryTimeout()
 
 void tst_QThreadPool::expiryTimeoutRace() // QTBUG-3786
 {
-#ifdef Q_OS_WIN
-    QSKIP("This test is unstable on Windows. See QTBUG-3786.");
-#endif
     ExpiryTimeoutTask task;
 
     TestThreadPool threadPool;