Get rid of the function-static main window, which would be destroyed
after QApplication if the test is run with a subset of test row (that
does not include the TestEnd state test).
Make the MainWindow a class member of the test class instead, and rename
it from "mainW" to "mainWindow" to avoid shadowing by "mainW" widgets in
other test functions.
Amends 55928821d1.
Task-number: QTBUG-99630
Pick-to: 6.2 6.3
Change-Id: I83efce5b54afc3a0027a7c0e63efee6a235af585
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
e8bf2c6eba