The coveralls service is a great webservice, but like the testsuite it's
better to have an alternative, which makes it possible to process it
locally. This is faster and doesn't even require a full push iteration.
gcovr is easy to get:
pip install gcovr
make test-coverage-report
Add the necessary compiler flags to the Makefile to actually make
coverage reports available locally, too.
The coverage files have to get cleaned every time before the test gets
run. Otherwise, it would sum up the coverage over multiple test runs.
