Add multitenancy testing infrastructure 👩‍🔧

[hug-dev]

With the various authenticators that we have and are going to have, it will be helpful to add automated testing to check that multitenancy is working securely.

As ideas for test cases:

1. The service should fail to run (under any authentication) if the socket folder has wrong permissions
2. Under direct authentication using the parsec-clients group, a client not in that group should not be able to connect
3. Under peer credential authentication (when ready):
   1. even a client not in parsec-clients should be able to connect
   2. one client can not access another client's keys

Regarding the infrastructure, these tests could be implemented in a separate GitHub Actions workflow (or more than one!), in a new top-level directory (eg multitenancy_tests).

[hug-dev]

Some work could be re-used from #218

[hug-dev]

Remainder to add the new tests added here