Make test pass on machines with many cores

Just starting 20 threads to test this won't cut it if
the machine you're testing on has an ideal thread count
of 16 or larger.

Change-Id: Icba8f00aa836fec6da41c71b318e9e17bdd47c0e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

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

@@ -799,7 +799,7 @@ void tst_QThreadPool::tryStartPeakThreadCount()
     CounterTask task;
     QThreadPool threadPool;
 
-    for (int i = 0; i < 20; ++i) {
+    for (int i = 0; i < 4*QThread::idealThreadCount(); ++i) {
         if (threadPool.tryStart(&task) == false)
             QTest::qWait(10);
     }