As suggested by Grzegorz, we can be more declarative in our
configuration of the tags the Entrypoint cares about

Here we add a tags method to the entrypoint class, and use that to
declare the tags an entrypoint is interested in (:core for the existing
Test::Instance, :core and :mattermost for Test::Integration::Mattermost)

We declare everything in specs/features that isn't in
specs/features/mattermost, to have the tag :core, though that clean up
better if we move the integration tests out of specs/features.