Add test coverage to the test suite

[foretspaisibles]

We now have a small test suite running on every push and we would like to see the test coverage. SBCL has some special extensions to do so.

Having the development/testsuite tool writing the details to test coverage on standard output would be good enough, or having a separate tool development/testcoverage doing so if that seems more practical.

Bonus points to pack the test coverage output in a ZIP file, store it in GitHub Actions, display coverage in a neat graph.