Disable metadata creation hack in blob service tests

Praefect contains a hack to create repository metadata when receiving a request for a repository. This was put in place to enable us to use the production code of Praefect in tests. That code requires database records for repositories to be present. Praefect piggy backed on Gitaly's tests for most of its RPC testing. Those tests broke with the new requirement as they generally just place the repositories in the storage directly rather than creating the repositories through the API which would allow Praefect to create the metadata records. This testing hack potentially hides problems in Praefect. As a first step towards removing it, this commit adds an option to disable the testing hack in the test server. This allows for asserting a given test no longer relies on the hack after being updated to create the repositories through the API.

Related to: #4019 (closed)