state transition testing relation events don't like remote unit zero

[dimaqq]

This test:

    ctx = testing.Context(DummyCharm, meta=META)
    secret = testing.Secret(tracked_content=BAD_SECRET_DATA)
    rel = testing.Relation(
        "rel", "intr",
        remote_app_data={"secret-uri": f'"secret://{secret.id}"'},
        remote_units_data={0: {}},
    )

    state_in = testing.State(relations={rel}, secrets={secret})
    state_out = ctx.run(ctx.on.relation_changed(relation=rel, remote_unit=0), state_in)

Yields ⚠️ WARNING ops-scenario.runtime.consistency_checker:_consistency_checker.py:122 This scenario is probably inconsistent. Double check, and ignore this warning if you're sure. The following warnings were found: 'rel_relation_changed' is implicitly using 0 as the remote unit. Consider passing remote_unit explicitly.

If I change the remote unit to 1 in both remote_units_data=... and remote_unit=... the warning disappears.

I think this means we're confusing the explicit remote unit 1 with the implicit/default.

Given that unit 0 is legal in Juju, we should fix this.