Use mocking in the tests

[jwallwork23]

In many cases it isn't necessary to actually build meshes, function spaces, or functions for testing. We should be able to use Python mocking instead. It should speed things up a bit.

https://docs.python.org/3/library/unittest.mock.html

[jwallwork23]

How about we mock the mesh class in as many cases as we can get away with? e.g., tests that exceptions get raised.