gitserver: refactor TestCleanupOldLocks

Created by: stefanhengl

The original test was too verbose as it checked for files that were unrelated to the test's objective. This meant we had to update the test even for unrelated changes, like adding a sgm.log file for failed sg maintenance runs.

We don't check for repos-stats.json anymore, because this is already covered in TestCleanup_computeStats.

Test plan

Unit tests pass