Update tests

[benbovy]

TODO:

• simplify the structure (no need of fixtures if it's just for the input parameters of a single test).
• test the actual logic implemented in compute_orographic_precip. Is there any ideal topography where we can compare the result against an analytical solution (e.g., with a gaussian pulse like shown in README)? Probably worth checking the paper.
• add tests for the two process classes in fastscape_ext. Testing OrographicPrecipitation could be very similar than the test for compute_orographic_precip. No need to test the whole logic of OrographicDrainageDischarge (its base class will be tested upstream in fastscape), just ensure that values are converted as expected.