test: optimize AutoReview tests

[paulbalandan]

Description

• In random tests, AutoReview tests currently take about 30s on its first run and normalizes to ~1s on subsequent runs. This optimizes the runs using a filename-based check instead of the expensive is_subclass_of() check.
• In CreateNewChangelogTest, moves the fetching of the whole repo to the workflow level.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide