Get parallel test CI working again

[ketch]

Since we switched to Github actions for CI, we only have serial tests running. To ensure the continued functionality and correctness of the parallel code, it's important to get the parallel tests going again. For reference, the old workflow to run them on Travis can be found here:

https://github.com/clawpack/pyclaw/blob/22403c81f7433ed0341ce95e139d055c342dd539/.travis.yml