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