[autotests] Give Xephyr more time to startup

The test is flaky during the wait for Xephyr. Giving it more time should
hopefully make it more reliable.

The better way would be to switch to the command line argument
-displayfd pipeFd

unfortunately the Xephyr on the CI system does not yet support this
command line argument.

autotests/test_xrandr_screens.cpp

@@ -82,7 +82,7 @@ void TestXRandRScreens::initTestCase()
     QVERIFY(m_xserver->waitForStarted());
     QCOMPARE(m_xserver->state(), QProcess::Running);
     // give it some time before we open the X Display
-    QTest::qWait(100);
+    QTest::qWait(500);
     // create X connection
     int screen = 0;
     s_connection = xcb_connect(qPrintable(id), &screen);