Add Test::Integration::Mattermost

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

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