Implement or add a testing framework

[max-veit]

As discussed in some previous meetings, it would be a good idea to collect our testing procedures so they can be run in a more automatic, systematic fashion (especially as regression tests, i.e. to make sure new changes don't break previously working functionality). Organizing these tests so that they can be run automatically usually requires quite a bit of extra code to set up and run the tests, and to check their results. Luckily, we don't have to reinvent the wheel here, as many so-called unit testing frameworks have already been implemented in Fortran. One that looks good is test-drive - it's pure Fortran and can be integrated into the project fairly easily (by copying in one .F90 file).

However, this also brings in the issues of third-party dependency management, which hasn't really been touched on in the TurboGAP code development AFAICT. There's also the issue of possible license incompatibility -- @mcaroba do you know whether we can include and redistribute the code (which is Apache 2.0 and MIT licensed) under TurboGAP's current ASL license?

Thoughts/suggestions welcome.