cleanup tests [qtbase]: inconsistent use of QTest::qWaitForWindowActive

TODO - improve description.

Clarify which is correct way of doing things and document in QTBUG-63987

touchWidget.show();
 QVERIFY(QTest::qWaitForWindowActive(&touchWidget));

VS

window.show();
window.requestActivate();
QVERIFY(QTest::qWaitForWindowActive(&window));

VS

pbar.show();
qApp->setActiveWindow(&pbar);
QVERIFY(QTest::qWaitForWindowActive(&pbar));

Regarding setActiveWindow: "Sets the active window to the active widget in response to a system event. The function is called from the platform specific event handlers." So it seems that it does not affect qWaitForWindowActive at all and should not be used like that, which is not the case with many tests.