ci: Hopefully decrease flakiness in `test-with-praefect`

This MR has the intent to decrease or fix flakiness in test-with-praefect, which is likely caused by an exhaustion of the connection pool. It should fix issues like https://gitlab.com/gitlab-org/gitaly/-/jobs/2496120168 and https://gitlab.com/gitlab-org/gitaly/-/jobs/2497314144. Furthermore, we should get better error reporting when we hit such a flaky test by not using the assert package in the testdb package anymore, which is seemingly incompatible with gotestsum.