We can currently reduce the number of times we specify the tag of a
given example group by moving them hoisting them up one level to the
containing `feature`

This was added as part of the previous filtering via RSpec::Config.  It
should have been removed previously as part of moving the logic to
specify the tags we wanted in Specs::Runner

It's adding additional potentially confusing behaviour to derive test
tagging from the file path, as it's not immediately apparent where that
lives when considering the tests in isolation.  Instead, just explicitly
tag.

* Remove the tagging of examples based on :file_path
* Explicitly tags scenarios as :core, or :mattermost

Here we convert Specs::Runner#rspec to use keyword arguments[1] and pass
named parameters rather than a pre-processed array of cli switches.
This allows parameter to cli logic to live just in Specs::Runner.

[1] https://robots.thoughtbot.com/ruby-2-keyword-arguments

Entrypoint is intended as a base class, so mention that in the comment.

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.

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

In doing some experimental testing of enabling the embedded mattermost
we found that the currently generated QA namespace was problematic in that
it used underscores when generating group names.  This is invalid for a
mattermost team name, underscores are illegal there.

Here we change to use hyphens as they are legal in team names.

Make use of a `gitlab-qa-sandbox` group if available

See merge request gitlab-org/gitlab-ce!14682