Establish Unit Tests via PyTest

[ax3l]

Let us move over the successful pyAMReX / ImpactX unit test system via PyTest and use it in WarpX to write more focused unit tests.

More productive than using Catch2 or other C++-only unit tests, because we often need a lot of data processing to validate results.

[ax3l]

co-assigning people that might be interested in this, from earlier conversations, to coordinate :)