Enable some tests in more-ci

[mbarbin]

Proposal to split the tests into parts that can serve different purposes.

Explanation:

In #21 I enabled more checks in the ci to target more os and compiler versions.

However at this time all the doc and tests items are grouped together into a single package named provider-tests.opam, which causes some issues to be run on all matrix combinations:

• it has too many dependencies (such as eio)
• it has brittle expect-tests (e.g. it uses ls and wc in the mdx doc, with formatting diff between ubuntu and macos).

As a result, for now these ci check only check that the code builds, but do not run any tests.

I thought it would make sense to split the tests and extract one new entry point that can be made to run on all checks matrix combinations. This is an issue to track this work.