Avoid sleeping 100ms in QProcessPrivate::drainOutputPipes()

There is no point in waiting 100 milliseconds after each iteration, as
all data that we may possibly read will be already in the pipe. We only
need to give the notifier thread a chance to inform us, which is best
achieved with a yield.

Task-number: QTBUG-41282
Change-Id: Id654b688246508494a5549c11900f9ad2957f192
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/io/qprocess_win.cpp

@@ -634,7 +634,7 @@ bool QProcessPrivate::drainOutputPipes()
         someReadyReadEmitted |= readyReadEmitted;
         if (!readOperationActive || !readyReadEmitted)
             break;
-        Sleep(100);
+        QThread::yieldCurrentThread();
     }
 
     return someReadyReadEmitted;