Testsuite Examples: Parallel test execution with pytest-xdist

Currently, tests are run sequentially. Which is feasible for unit tests, but for the examples tests which can take quite a bit of time, parallel execution would be great.
Though, we have to take into account that each example might use several processes through ORCA as well.

https://qxf2.com/blog/xdist-run-tests-parallel-pytest/