Tests: Improve isolation for randomized multisite tests

[sirreal]

Summary

• Removes hard-coded site-ID, directory-existence, lazy-load queue, external-DNS, and wp_installing state assumptions from multisite fixtures.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 randomized multisite-group runs passed; each completed 763 tests and 1,805 assertions with two expected skips.
• The continuation covered seeds 1789073001 through 1789073050; maximum authoritative PHPUnit time was 67.065 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