Tests for Courant Number diagnostics #676
Description
As Josh pointed out here:
Originally posted by @JHopeCollins in #675 (comment)
we don't currently have any unit-tests for the Courant number calculation (or many of the other diagnostics)
Maybe we could use this as inspiration:
https://github.com/firedrakeproject/asQ/blob/master/tests/utils/test_diagnostics.py