Here we add a new entrypoint for the test suite,
Test::Integration::Mattermost. It is to ensure that mattermost
integration is working when enabling the embedded mattermost of
gitlab-omnibus
It runs all the example groups from Test::Instance, in addition to any
groups tagged :mattermost
* Extracts a common entrypoint class from Test::Instance as
Scenario::Entrypoint, and uses that as the base for Test::Instance and
Test::Integration::Mattermost
* RSpec groups defined in `specs/features/mattermost/**_spec.rb` are
tagged with :mattermost so they can be filtered out of the default
run.
* Tests tagged :mattermost are filtered out by default, and are
un-filtered by Test::Integration::Mattermost
