Make it super-easy to collect code coverage for test suite

[fingolfin]

We should make it super easy to compute code coverage results for the test suite. E.g. there could be a make target coverage, or perhaps cov-testinstall and cov-testandard, and thes would start GAP with the right options, run the selected test suite, dump everything into a specific place, run profiling on it to create HTML, and finally tell the user where the index.html is.

[ChrisJefferson]

If anyone wants to work on this, there is a basis vagrant box here, which I have used to profile GAP and C (it isn't really in a finished state as it is). https://github.com/ChrisJefferson/GAPCoverage

[fingolfin]

With the codecov setup, I am personally mostly happy now, and we might consider this as resolved.

Of course it would be nice if we next could also provide a ready-made solution to package authors, so that they can trivially setup travis and codecov for their packages... Something like my GitHubPagesForGAP just for packages. But that's really a separate issue.

[fingolfin]

I am actually satisfied with the current state using codecov; we already pushed this out to dozens of packages, too.