Tests: Restore Ajax hooks for each test

[sirreal]

Summary

• Restores the Ajax callback baseline during every test setup so a preceding test cannot permanently remove callbacks required by another case.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 consecutive fresh randomized Ajax-group runs passed on the final combined campaign head; each completed 190 tests and 1,212 assertions with one expected skip.
• Exact seeds 1789050001 through 1789050100 all completed within the 30-second hard bound.
• The final gate also covered the non-querying locale restoration carried by the l10n PR; the Ajax callback change itself had already passed the original 50-run isolated gate.
• 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