tst_QGL: Use qWaitForWindowExposed()

Instead of an unconditional wait.

Change-Id: I360cd665c10c3fd4f936c95ef04957ae1c49c0de
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

tests/auto/opengl/qgl/tst_qgl.cpp

@@ -923,8 +923,7 @@ void tst_QGL::partialGLWidgetUpdates()
     widget.setFixedSize(150, 150);
     widget.setAutoFillBackground(autoFillBackground);
     widget.show();
-
-    QTest::qWait(200);
+    QVERIFY(QTest::qWaitForWindowExposed(&widget));
 
     if (widget.format().doubleBuffer() != doubleBufferedContext)
         QSKIP("Platform does not support requested format");