Tests: Improve isolation for randomized admin tests

[sirreal]

Summary

• Resets admin menu registries and plugin-dependency statics, removes temporary MU-plugin fixtures, isolates list-table screens, and makes updater open_basedir tests account for an external Composer vendor directory.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 randomized admin-group runs passed; each completed 1,024 tests and 2,721 assertions with ten known warnings and one skip.
• The continuation covered seeds 1789072001 through 1789072050; maximum authoritative PHPUnit time was 90.776 seconds.
• 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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