Tests: Improve isolation for randomized media tests - #13190

Summary

• Resets loading-optimization state before each test and replaces a live crop URL with a local fixture.
• Accepts generated caption-ID suffixes and derives expected attachment names from the files actually uploaded.
• Restores the incoming HTTPS server value after attachment tests so a class-level fixture cannot capture leaked request state.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• Minimized seed 1789074025 to the HTTPS-mutating attachment test followed by the attachment-link class fixture, then replayed the full failing seed after the fix.
• 100 consecutive fresh randomized media-group runs passed on the combined campaign branch after resetting the counter; each completed 842 tests and 2,220 assertions with 52 expected skips.
• Exact seeds 1789084001 through 1789084100 all exited cleanly without exclusions or replays; maximum PHPUnit time was 33.552 seconds.
• All three relevant changed file trees match this isolated PR branch, and the HTTPS restoration patch ID matches the gated commit.
• PHP syntax checks and git diff --check passed.

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