allow testing peer and subordinate relations

At the moment the tester assumes the relation we're testing is regular. There is no way to configure it to use other relation types.

This is not an interface concern: interfaces are independent of the relation type that supports them.
So the Tester should be made smart enough to infer the relation type from the charm metadata (or accept charm-facing config to the same effect).

Start by tracing the Tester and how it currently selects regular relations, then inspect how charm metadata is exposed to the pytest plugin. Define the supported peer and subordinate relation configuration and verify that interface protocol checks work for each relation type without changing interface behavior.

Written by the indexing model from the issue text.