simplify the testing API

[PietroPasotti]

as it stands, the testing API is too generic.

you write:

t = Tester(state_in=scenario.State)
t.run("foo-relation-changed")

whereas:

1. the only part of the state that the test should be concerned about is a single relation
2. the relation endpoint, and therefore the name of the event we are running, is not owned by the test either, but by the charm being tested.

Proposal:

consider cleaning up the API accordingly

from interface_tester import Relation
t = Tester(relation_in=Relation(remote_app_data={...}))
t.run_relation_changed()
...

This requires:

0. replacing state_in with relation_in in Tester; use relation_in to construct the usual state_in
1. adding a dedicated Relation subclass that gets rid of the 'endpoint' argument so the user doesn't even need to think about this. Because unfortunately, endpoint is the only mandatory arg to scenario.state.Relation :D
2. replacing generic run() with run_relation_changed and similar for other event types.