Tests: Improve isolation for randomized rewrite tests

[sirreal]

Summary

• Flushes rewrite rules after initial taxonomy registration and clears leaked HTTPS server state during rewrite-suite teardown.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 randomized rewrite-group runs passed; each completed 1,393 tests and 2,826 assertions.
• The continuation covered seeds 1789032001 through 1789032050, all with clean exits.
• 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

[lancewillett]

Updated this branch with a non-rewriting merge of current trunk (1339550).

Validation completed:

• PHPCS on both changed files
• Focused rewrite and conditional-query tests: 213 tests, 955 assertions
• Randomized rewrite group with seeds 13193 and 13195: 1,393 tests, 2,826 assertions each
• git diff --check

[github-actions]

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[github-actions]

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Props jonsurrell, lancewillett.

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[lancewillett]

https://core.trac.wordpress.org/changeset/63594