Directly call ERT to run the tests
Add a custom test runner to enable test selection

The test runner just reads a selector expression from the first argument
Remove ERT Runner from Cask file
Don't allow failures on Emacs snapshot anymore

It doesn't fail anymore without ERT Runner
Merge branch 'no-ert-runner'

* no-ert-runner:
  Don't allow failures on Emacs snapshot anymore
  Remove ERT Runner from Cask file
  Add a custom test runner to enable test selection
  Directly call ERT to run the tests
Clarify test requirements
Bump sphinxcontrib-emacs requirement
Add section about unit testing to contrib guide
Rebuild Texinfo manual