Only create the replica's record in SetGeneration

SetGeneration is used to set a replica's generation after it has been replicated to. In the past, it also created a record for the repository if it was missing one. This has been abused in the tests to create records for the repositories while setting a desired generation for a replica. Repository creation logic has since expanded, and the tests should also create repositories by using CreateRepository. SetGeneration should never create a repository record, so this MR drops the behavior now that tests have been updated to properly create the repository records they expect.

This MR also amends tests that relied on this behavior to explicitly create the repository records they expect.