Expand the testing efforts

[nift4]

• Current state: testing util methods are written, permission checks are very well-tested, as well as scan behaviour being tested at good/okay level. Then, create() is tested okay, mostly missing subdirectory and hidden file related tests.
• But we should test every method, or at least the suspicious ones, in any combination of valid arguments on any Android emulator.
• And we should try Miko's unit test harness and make the test suite green with it, to automate testing on 12 Android versions.
• Bonus points when testing on some OEM skin I have access to or can convince someone to have access to. Good thing: It matters way less on Android 11 or later, because MediaProvider is a mainline module there so OEMs cannot possibly break it.