Use branch name to run a single RSpec tag before all other tests

Proposal

Use a special branch name to run any single RSpec tag before all tests.

One use-case: You can temporarily tag the set of specs you are interested in right now, e.g.:

it 'tests a thing', :mkozono do
  ...
end

Branch name: my-branch-name-tag-mkozono

We need some kind of delimiter or way to detect a desired tag so we don't detect everything. But also it shouldn't be a huge deal if we get occasional false positives, e.g. fix-git-tag-issue will look for RSpec tag issue.

If this is implemented, it would supercede https://gitlab.com/gitlab-org/gitlab-ee/issues/11208