playlist: refactor tests

[snejus]

Part of #6807.

• Refactors test/plugins/test_playlist.py to use pathlib.Path as the main path abstraction instead of building paths manually with os.path.join.

• Centralizes playlist test setup by introducing shared helpers like write_absolute_playlist(), write_relative_playlist(), and reusable path attributes such as c_track_path and nonexisting_track_path.

• Simplifies fixture data creation by using self.add_album(...) in a small loop instead of constructing test items manually.

• Architecturally, this change makes the test base class responsible for common path definitions, playlist file generation, and initial library setup. The individual test classes now mostly declare only the playlist configuration they care about.

• High-level impact: the playlist tests become more consistent, less repetitive, and easier to extend, while keeping behavior the same.

• It also makes path handling more explicit and type-safe in the tests by treating filesystem paths as Path objects and only converting to str where config values require it.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.83%. Comparing base (8ddae79) to head (7a8c529).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beets/util/extension.py	70.00%	1 Missing and 2 partials ⚠️
beets/importer/tasks.py	89.47%	1 Missing and 1 partial ⚠️
beets/util/__init__.py	87.50%	1 Missing and 1 partial ⚠️
beets/util/hidden.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6819      +/-   ##
==========================================
- Coverage   74.85%   74.83%   -0.02%     
==========================================
  Files         163      163              
  Lines       20970    20986      +16     
  Branches     3300     3304       +4     
==========================================
+ Hits        15697    15705       +8     
- Misses       4519     4523       +4     
- Partials      754      758       +4     

Files with missing lines	Coverage Δ
beetsplug/fetchart.py	74.18% <100.00%> (ø)
beets/util/hidden.py	54.16% <83.33%> (+6.54%)	⬆️
beets/importer/tasks.py	90.87% <89.47%> (-0.29%)	⬇️
beets/util/__init__.py	80.35% <87.50%> (-0.33%)	⬇️
beets/util/extension.py	76.47% <70.00%> (-4.18%)	⬇️

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[Copilot]

Pull request overview

grug see this PR make playlist plugin tests use pathlib.Path more, and share setup helpers so test code repeat less.

Changes:

• move many test-owned paths to Path objects and compose with /
• centralize playlist file writing in base test case (write_absolute_playlist, write_relative_playlist)
• simplify library seed data by using self.add_album(...) loop