Issue #2988770: convert legacy D6/7 tests to PHPunit.

- First step: kernel test for Logger.
- Passing Browser test which test nothing and sets up/tears down cleanly.
- Ported and expanded ControllerTest::testDbLog().
- Ported and adapted testDbLogClear.
- Ported and adapted testFilter.
- Improve quality with strict types in watchdog to catch more bugs.