Better coverage collection method

[rollbear]

My attempts to use gcov always resulted in empty reports, thus resorting to kcov which was easy to get working, but it reports weird missed lines and thus way pessimistic results. Any help on getting gcov to work, and also getting coverage on Windows, would be much appreciated.

[rollbear]

Fixed it. I had (unsurprisingly) misunderstood a lot about gcov and codecov.io uploads.