ci: enable coverage reports for test job

Enable coverage support in the CI pipeline to display in MR as well as in repository analytics.

Passing the --coverage option to the compiler and running gcovr after make test produces the summary gitlab is looking for with the regex.

We might want to tweak the gcovr run later on to only include the actual libraries we test and some example code to have accurate reports, but for now this is no problem.

Signed-off-by: Stefan Schmidt stefan.schmidt@huawei.com