Fix image mixins tests

[JonnyWong16]

Description

• Enable Playlist art mixins tests. Editing Playlist art requires PMS >= 1.24.5.
• Unlock images after edit image test to restore pre-test state.
• Mark all image mixins tests as xfail.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable