Tests: Restore locale state after each test

[sirreal]

Summary

• Synchronizes the translation controller with the restored site locale during base teardown and restores the original WP_Locale object after locale-switcher tests.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 randomized i18n-group runs passed; each completed 267 tests and 809 assertions with one known warning.
• The continuation covered seeds 1789037001 through 1789037050, all within the 30-second hard bound.
• 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

[github-actions]

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