CI: Run tests in parallel to make overall execution faster
While we're using tailored containers to run the testsuite nowadays to save time spent on installing dependencies, I still think there is quite some time to gain in case we would run the testsuite in parallel, instead of linar, each file after each other, as currently.
I've made some tests which look promising.
Before investing more time do get this to work, I wanted to check first if this is desirable. If so, I'll propose the needed changes.