Revive tests and improve the tests runner

[Niols]

This PR is here to bring back the tests that we have (and that aren't that much to begin with). There will be several parts to this:

• Update the CI so that it runs tests again.
• Update the Dune configuration so that dune test does what we expect.
• Update the Nix build so that it also runs the tests.
• Fix the test runner to follow changes in Morbig.

Edit: mentioned later in the PR:

• The comparison of ASTs should be made without comparing the locations. (This is maybe already the case but it should then be documented.) Edit: Introduce an AST equality that ignore the locations #31
• The printing of ASTs in JSON should hide the locations in the test runner. In general, we should provide two different serialisers, one with and one without locations. Alternatively, we could rework Morsmall to provide two ASTs, one with and one without locations; this should be easy to do with a well-made functor. Edit: Introduce a JSON serialisation that ignores locations #32
• Currently, in CI, we get the logging output of the tests runner, but this does not include the artifacts. We should upload them to allow inspection. Edit: CI should update the test artifacts #33

[Niols]

Yay, the CI is now failing all over the place \o

[Niols]

The test runner is now looking much better. It compiles, the code is cleaner and its error reporting is also much better. Still, a few things remain before we can do anything with it:

• The comparison of ASTs should be made without comparing the locations. (This is maybe already the case but it should then be documented.)
• The printing of ASTs in JSON should hide the locations in the test runner. In general, we should provide two different serialisers, one with and one without locations. Alternatively, we could rework Morsmall to provide two ASTs, one with and one without locations; this should be easy to do with a well-made functor.

[Niols]

Additional thought:

• Currently, in CI, we get the logging output of the tests runner, but this does not include the artifacts. We should upload them to allow inspection.

[Niols]

I think that, at this point, we can consider that the tests are fixed indeed. CI is unpleased because Morsmall is broken in many ways, but that is not so much the problem of the tests. We do need to fix these last three points but I will open issues for all of them before merging this PR.