Measure test coverage

We have a test suite in Weston (soon looking quite different under the hood due to !287 (merged)), we assume we are poor with test coverage, but how poor are we exactly?

Experiment with Meson's coverage support and see what we can get out.

Gitlab seems to be very limited with its coverage reporting support, but I'm thinking of something more like... sunburst charts where the center is the project root directory with the total coverage percentage, and directories and files fan out from it, having source code files at the leaves. With color coding for coverage. So on a glance you can see that fullscreen-shell/* is completely untested but has little code, much of clients/* is never exercised but is not that much code either, libweston/* is (hopefully!) a little better and contains a huge load of code, and so on.

How to publish a chart from e.g. a CI run is a good question.

What would be most interesting is how each MR changes the coverage numbers, but I have no idea how that could be implemented.

It would also be nice to compare coverage reports, so that when you add a new test, you can see exactly which bits of code started being tested.