Tests: Isolate user fixtures from randomized execution

[sirreal]

Summary

• Avoids mutating shared user IDs, refreshes cached capability fixtures, scopes INFORMATION_SCHEMA lookup to the active database, and isolates the constant-defining capability test.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 50 consecutive randomized user-group runs passed; each completed 1,341 tests and 4,528 assertions.
• 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