IDEA: add comments to select tests for training purposes

[truth-quark]

Following skillshare, it's apparent the setup approach & intent of tests is less obvious to project newcomers (& those new to mock). One question asked about why the tests used these mocks:

um2nc-standalone/test/test_um2netcdf.py

Lines 200 to 201 in 33d0dc5

	mock.patch("mule.load_umfile"),
	mock.patch("umpost.um2netcdf.process_mule_vars") as m_mule_vars,

Which cover this code:

um2nc-standalone/umpost/um2netcdf.py

Lines 468 to 469 in 33d0dc5

	ff = mule.load_umfile(str(infile))
	mv = process_mule_vars(ff)

The 2 mock shortcut makes load_umfile() do nothing & simulates process_mule_vars() returning data. An alternative is mocking load_umfile() to return a mock mule object that can be "opened" in process_mule_vars() & return data values. That's clearly more complex than the current approach.

Therefore, it's potentially worth over documenting a couple of tests to provide training hints.

Downsides:

• Once mule calls are extracted & process() takes data structure args, mocking isn't required.
• The comments "rot"...