Unit tests for Batch using mock objects

[xop5]

Use unittest mock objects to test the batch pool creation

[natemcintosh]

This looks great! So happy to finally have some tests for this library. That said, it looks like we still need to set up CI for the tests.

I'd recommend something like this since both projects use poetry. That said, it'll need a little work to remove the PyRenew specific parts.

[xop5]

#123