QFileDialog: Stabilize tests.

The init()/cleanup() code in tst_qfiledialog and tst_qfiledialog2
currently differs and fails to clean up the settings file
since it only removes the legacy settings under the Qt group
and instantiates a new QFileDialog while the QSettings class
is still in scope. Also, it has no means of clearing the
setLastVisitedDirectory(), which causes the
tst_QFiledialog::completer() and tst_QFiledialog::history() tests
to interfere, leaving the settings in an invalid state.
tst_qfiledialog2 does not use QStandardPaths::setTestModeEnabled(().

- Ensure the last visited URL is always clean by
  making QFileDialogPrivate::setLastVisitedDirectory()
  static and calling it from init().
- Introduce a cleanupSettingsFile() function to the tests that
  cleans both groups and call it from initTestCase() and cleanup()
  to ensure a clean state.
- Add QStandardPaths::setTestModeEnabled() to tst_qfiledialog2.

Fixes sporadic test fails when executing
tst_QFiledialog::completer() and tst_QFiledialog::history()
in a sequence.

Task-number: QTBUG-45764
Change-Id: I24de3caabf77be067b385d64ff11b7a07fe12b72
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>