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

• 50 consecutive randomized Ajax-group runs passed; each completed 190 tests and 1,212 assertions.
• 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