tst_QWindow::isActive is flaky on Ubuntu in CI

********* Start testing of tst_QWindow *********
Config: Using QtTest library 5.11.0, Qt 5.11.0 (x86_64-little_endian-lp64 shared (dynamic) debug build; by GCC 5.4.0 20160609)
PASS   : tst_QWindow::initTestCase()
QINFO  : tst_QWindow::isActive() child: QWindow(0x7ffd8c2c4890)
QINFO  : tst_QWindow::isActive() dialog: QWindow(0x7ffd8c2c48f0)
QINFO  : tst_QWindow::isActive() window: QWindow(0x7ffd8c2c4830)
QINFO  : tst_QWindow::isActive() focusWindow: QWindow(0x7ffd8c2c48f0)
FAIL!  : tst_QWindow::isActive() Compared pointers are not the same
   Loc: [tst_qwindow.cpp(885)]
PASS   : tst_QWindow::cleanupTestCase()
Totals: 2 passed, 1 failed, 0 skipped, 0 blacklisted, 15431ms
********* Finished testing of tst_QWindow *********