tests: Exercise the new multiple cache support

This adds a new test for parsing artifact cache configuration, which
calls the helper function from the 'artifactcache' module directly
rather than trying to assert based on blind push and pull commands
whether or not we got the complex precedence rules exactly right.

This means frontend push/pull tests no longer need to be so thorough
about testing precedence but they are instead expanded to assert that
multiple caches work correctly.