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

Fix a feature spec failing only on MySQL

Closes #39003

See merge request gitlab-org/gitlab-ce!14811

Add support for markdown preview to group milestones

Closes #30241

See merge request gitlab-org/gitlab-ce!14806

fix the import :milestone from adding the group_id

Closes #35580

See merge request gitlab-org/gitlab-ce!14657

Use Gitaly's RepositoryService.HasLocalBranches RPC

See merge request gitlab-org/gitlab-ce!14753

Add sort runners on admin runners

Closes #38720

See merge request gitlab-org/gitlab-ce!14661

Move callback route(`google_api/auth/callback`) for Oauth under `-`

Closes #38911

See merge request gitlab-org/gitlab-ce!14802

Simplify project page

Closes #37399 and #38839

See merge request gitlab-org/gitlab-ce!14669

Developer can create/edit/delete milestones

See merge request gitlab-org/gitlab-ce!14797

QA group name, use hyphens rather than underscores

See merge request gitlab-org/gitlab-ce!14786

Enable eslint in star and remove from main.js

See merge request gitlab-org/gitlab-ce!14795

Use ES6 modules in labels and labels manager

See merge request gitlab-org/gitlab-ce!14794

Add image discussion docs

Closes #38879

See merge request gitlab-org/gitlab-ce!14743

Refactor the Development documentation, and divide the Testing documentation into multiple pages

See merge request gitlab-org/gitlab-ce!14512

Update Kubernetes Helm chart docs

Closes #38828

See merge request gitlab-org/gitlab-ce!14717

Add explicit test to test resolved discussion toggle content

See merge request gitlab-org/gitlab-ce!14796

Decreases z-index of select2 to a lower number than our navigation bar

Closes #36160

See merge request gitlab-org/gitlab-ce!14768

Remove not selector increasing specificity of rules

Closes #38958

See merge request gitlab-org/gitlab-ce!14789

Locked issues / mrs docs

Closes #38880

See merge request gitlab-org/gitlab-ce!14770