Add a Github Action CI workflow to run all the tests

[shym]

This PR adds a new CI workflow that runs all the tests in an action.

The goals are to:

• test the installation of the OPAM package (not tested by OCaml-CI) to make sure we are ready before we release,
• test the noisy tests (that are not attached to @runtest because they are noisy),
• provide a fallback for when OCaml-CI is overwhelmed.

This should provide a more reliable way to avoid broken states than what #327 tried to introduce.