repository: Fix slow tests exceeding lockfile cleanup grace period

When cleaning up lockfiles, we have a grace period of 15 minutes before we remove them. This is done such that we don't remove any that have just been created by Git.

Now some tests in the Repository service test this by creating their own lockfiles. They do that by using a global freshTime variable that is getting initialized with time.Now(). This means that "fresh" is whatever had been fresh when the test executable started, where on macOS there can be a delay of more than 15 minutes until we finally execute these tests. The result is that "fresh" is "stale" and thus tests are flaky.

Fix this by instantiating times locally in the testcases.