Reimplement fluidsim-test by just calling pytest

I don't use fluidsim-test anymore but directly pytest, which is just more convenient.

A problem is that there are differences between fluidsim-test and pytest (stdout capture, test discovery).

Therefore, I think we need to reimplement something like fluidsim-test but with just using pytest programmatically, as explained here https://github.com/pytest-dev/pytest/issues/2544#issuecomment-312742145