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 build
  dir accessible by processes running 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>`.