Fix running test when building against Qt 6 and enforce it

• unittests/qml: post-process QML files for Qt 6 to remove import versions
• unittests/qml: {WebEngineLoadRequest -> WebEngineView}.Load*Status
• webbrowser/qml-qt{5,6}: pass URL, not path, to FolderListModel
• unittests/qml/tst_HistoryViewWide: correct expectation for close button
• app/favicon-fetcher: explicitly tell Qt not to follow redirects
• unittests/favicon-fetcher: follow rename error{ -> Occurred}
• unittests/qml/tst_NewTabView: wait for button to render before clicking
• webbrowser/bookmarks-folder-model: emit countChanged() more frequently
• HistoryViewWide: correctly handle modelChanged while activeFocus
• unittests/favicon-fetcher: increase timeout on shouldCancelRequests()
• unittests/qml/tst_BookmarksView: wait for BookmarksView to render
• BookmarksFoldersView: don't set size at both Loader and loaded item
• d/rules: start enforcing Qt 6 tests

Fixes: #605 (closed)