KoFileDialog rather belongs to kowidgetutils than kowidgets

KoFileDialog belongs to "Tier 0"

Also move filedialogtester to widgetutils:

• remove all i18n - not needed for the test
• rename to kofiledialogtester
• move to tests/ subdir
• remove main.cpp which is too general file, place main() in KoFileDialogTester.cpp

Differential Revision: https://phabricator.kde.org/D360