Tests: Preserve default excluded groups when using --exclude-group

[ArkaPrabhaChowdhury]

Summary

Preserves the default excluded PHPUnit groups when an additional --exclude-group argument is passed.

Trac

https://core.trac.wordpress.org/ticket/34185

Testing

• Run npm run test:php -- --filter Tests_PHPUnit_GroupFilters
• Run npm run test:php -- --exclude-group import --filter test_populate_site_meta
• Run npm run test:php -- --group ajax and verify ajax tests are no longer skipped by default

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