autotag: refactor autotag tests to use single comprehensive test

Consolidate multiple granular test methods in ApplyTest into a single
comprehensive test that validates all applied metadata at once. This
improves test maintainability and clarity by:

- Replacing ~20 individual test methods with one data-driven test
- Using expected data dictionaries to validate all fields together
- Removing ApplyCompilationTest class (covered by va=True in main test)
- Keeping focused tests for edge cases (artist_credit, date handling)
- Switching from BeetsTestCase to standard TestCase for speed
- Adding operator import for efficient data extraction

The new approach makes it easier to validate all applied metadata at once.

Author: snejus