Enable parallelized test runs in coverage test

Currently, the coverage test runs single-threaded only which blocks the runner or quite some time. Reason for that is that GCOV writes to the same file regardless of threads/subprocesses. There is an option, though, to make it work with multi-threading which may need some change to the setup of the test jobs.

Subtasks:

• enable parallelization in the schedule coverage run
• check runtime impact
• if runtime impact and implementation effort is not too much, coverage could be measured as part of the MR test jobs