Refactor coord mocks when #63/lat long fixes is merged

[truth-quark]

When #63 is merged, I'll refactor the test_process_...() test setup to remove the coord mock & ensure the prod code is called.

Removing the fix_level_coord() & cubewrite() mocks can be handled in separate PRs (working on a model of gradually improving tests as um2nc is improved).

Originally posted by @truth-quark in #58 (comment)

To clarify tasks:

Replace buggy implementation with DummyCube with no coord attrs:

um2nc-standalone/test/test_um2netcdf.py

Lines 73 to 79 in 120dc47

	def set_default_attrs(cube, item_code: int, var_name: str):
	"""Add subset of default attributes to flesh out cube like objects."""
	cube.__dict__.update({"item_code": item_code,
	"var_name": var_name,
	"long_name": "",
	"coord": {"latitude": 0.0, # TODO: real val = ?
	"longitude": 0.0}, # TODO: real val

Remove these mocks & the tests should pass:

um2nc-standalone/test/test_um2netcdf.py

Line 160 in 120dc47

mock.patch("umpost.um2netcdf.fix_latlon_coords"),

[truth-quark]

From exploratory work on branch 69/refactor-cube-mocks:

Before:

um2nc-standalone/test/test_um2netcdf.py

Lines 73 to 94 in 120dc47

	def set_default_attrs(cube, item_code: int, var_name: str):
	"""Add subset of default attributes to flesh out cube like objects."""
	cube.__dict__.update({"item_code": item_code,
	"var_name": var_name,
	"long_name": "",
	"coord": {"latitude": 0.0, # TODO: real val = ?
	"longitude": 0.0}, # TODO: real val
	"cell_methods": [],
	"data": None,
	})
	section, item = um2nc.to_stash_code(item_code)
	cube.attributes = {um2nc.STASH: DummyStash(section, item)}
	@pytest.fixture
	def air_temp_cube():
	# data copied from aiihca.paa1jan.subset file
	name = "air_temperature"
	m_air_temp = mock.NonCallableMagicMock(spec=iris.cube.Cube, name=name)
	set_default_attrs(m_air_temp, 30204, name)
	return m_air_temp

After:

um2nc-standalone/test/test_um2netcdf.py

Lines 73 to 78 in 43a541e

	@pytest.fixture
	def air_temp_cube(lat_river_coord, lon_river_coord):
	# details copied from aiihca.paa1jan.subset file
	air_temp = DummyCube(30204, "air_temperature",
	coords=[lat_river_coord, lon_river_coord])
	return air_temp