tests: Add possibility to generate a code coverage report

To generate such a report:
* Build sources with CMAKE_BUILD_TYPE=Coverage and install the binaries.
* Run the script: coverage.sh prepare <build-dir> to make the files in
  the build dir accessible by processes which run during tests.
* Run the tests with GENTLY_KILL=1 environmental variable. This will ensure
  that all the binaries dump coverage data on exit.
* Generate a HTML report: coverage.sh generate-html <build-dir> <out-dir>.