Generate test coverage report in CI

This is the simplest way to generate a test code coverage report in CI. You can easily run the same on your localhost.

The report can be found by browsing to the job build-native-meson-default-options artifacts and build-weston-build-native-meson-default-options -> meson-logs -> coveragereport -> index.html.

But before merging this, I think we should have some plans for how to publish the results. Autogenerated link to the index.html or above somehow? Put stuff into Weston Pages?

I've exposed the results the same way as we expose documentation preview.

And what about comparing coverage reports between two branches like master and a MR?

Left for later, so keep #309 open.

Related to: #309