Parallel execution of the system tests

[MakisH]

Our system tests currently run sequentially, in one GitHub Actions workflow and from one execution of the systemtests.py:

1. Trigger GitHub Actions workflow
2. Trigger systemtests.py with a test suite
3. Start Docker Compose services for each specific test combination

We could parallelize at any of these levels:

1. Use a matrix in the GitHub Actions workflow and trigger different test suites in parallel.
   • Advantage: potentially using more hosted runners and getting more refined output on GitHub
   • Disadvantage: we can only parallelize on GitHub (which is generally what we need).
2. Parallelize the systemtests.py so that it calls Docker Compose in parallel.
   • Advantage: Runs also locally
   • Disadvantage: Will be messy, not portable to other runners.

@PranjalManhgaye what do you think?