Use gcovr for Test Code Coverage Reports

This MR adds some CMake guff to be able to run gcovr to evaluate how many lines of our code have been triggered by the testing procedures we have.

Right now the implementation is rather limited

• we only look at src/core
• we only include text_core unit tests
• the coverage is run on a separate job

Ideally I would like to

• have separate Debug and Release builds for testing and performance tests
• have coverage always active for Debug builds and collect coverage results from test_core and test_module

but I'm open for discussions.