…d modules

[Why]
rabbit_access_control checks that the auth backend module is provided by
`rabbit` or an enabled plugin. Some testsuites use a local auth backend
module for tests purpose that do not come as a plugin. We need a
mechanism to allow that.

[How]
We introduce a new `test_auth_backends` application envoironment
variable that testsuites can used to declare their valid auth backend
modules.

[Why]
rabbit_access_control now ensures all configured auth backend modules
are provided by `rabbit` or an enabled plugin. This is not the case for
this testsuite's own auth backend.

[How]
Declare `rabbit_auth_backend_mqtt_mock` as a test auth backend module.

[Why]
It didn't handle them before and crashed later when it assumed the
return value was a list.

…ch()`

[Why]
Hopefully, this fixes some test flakes in CI.