multipleToplevelFocusCheck() occasionally failed on XCB because
QApplication::activeWindow() was nullptr immediately after
qWaitForWindowActive() returned true.
This patch replaces QCOMPARE on QGuiApplicaiton::activeWindow() with
QTRY_COMPARE in order to continue spinning the event loop until the
condition has been met. It adds QWidget::activateWindow after show()
to ensure focus is acquired also when another window has received
focus in the meanwhile.
Pick-to: 6.2 6.4
Change-Id: If84eb8b77c5a6b16af271334a1fe5eb92c05644b
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>