Tests: Improve isolation for randomized media tests

[sirreal]

Summary

• Resets loading-optimization state before each test, replaces a live crop URL with a local fixture, accepts generated caption-ID suffixes, and derives expected attachment names from actual uploads.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 50 consecutive randomized media-group runs passed; each completed 842 tests and 2,220 assertions with 52 expected skips.
• Replayed the original failing seed and focused contaminator/consumer cases during diagnosis.
• PHP syntax checks passed for every changed PHP file.
• git diff --check passed.

Trac: https://core.trac.wordpress.org/ticket/65893