Improve tests for swh-alter

The semantic of the tests assumed that swh-alter had been foked from swh-py-template. This was not true as files were copied instead. We now use the main swh-py-template repository and lunar/swh-py-template.

We now ensure–by checking removed swhids and swhids referenced by the removal set-that the “Marking candidates used elsewhere” step of the selection algorithm work. This would not have been the case without the record_references storage proxy configured in c6cf71e3.

The objstorage tests that were done on the fork now has to be done on the initial repository to ensure a sample of Content objects big enough to matter.

We also test what happens when we remove all origins at the same time.