Implement test_fix_pressure_levels_reverse_pressure()

[truth-quark]

There is a testing gap from early work on #92, implenting test_fix_pressure_levels_reverse_pressure(). Problems occurred with testing at the boundary of um2netcdf.py & iris.util.reverse(). Instead of blocking other tests from merging, the team agreed to defer the fix to this issue.

Testing didn't cover this code branch calling an iris function:

um2nc-standalone/umpost/um2netcdf.py

Lines 757 to 761 in f6ba005

	if pressure.points[0] < pressure.points[-1]:
	# Flip to get pressure decreasing as per CMIP6 standard
	# NOTE: returns a new cube!
	# TODO: add an iris.util.monotonic() check here?
	return iris.util.reverse(cube, 'pressure')

Problems are listed in the test code (see also #94):

um2nc-standalone/test/test_um2netcdf.py

Lines 789 to 821 in f6ba005

	@pytest.mark.skip
	def test_fix_pressure_levels_reverse_pressure(get_fake_cube_coords):
	# TODO: test is broken due to fiddly mocking problems (see below)
	m_pressure = mock.Mock()
	# m_pressure.ndim = 1
	_add_attrs_points(m_pressure, [0.000001, 1.000001])
	extra = {"pressure": m_pressure}
	cube = get_fake_cube_coords(extra)
	# cube.ndim = 3
	# TODO: testing gets odd here at the um2nc & iris "boundary":
	# * A mock reverse() needs to flip pressure.points & return a modified cube.
	# Creating a mock to verifying these attributes is unproductive.
	# * Using the real reverse() requires several additional cube attributes
	# (see commented out ndim etc above). It requires __getitem__() for
	# https://github.com/SciTools/iris/blob/main/lib/iris/util.py#L612
	#
	# TODO: this leaves a few options:
	# * ignore unit testing this branch (rely on integration testing?)
	# * replace iris with an adapter?
	# * fix/refactor the function later?
	#
	# The test is disabled awaiting a solution...
	with mock.patch("iris.util.reverse"):
	mod_cube = um2nc.fix_pressure_levels(cube)
	assert mod_cube is not None
	assert mod_cube != cube
	c_pressure = mod_cube.coord('pressure')
	assert c_pressure.attributes["positive"] == "down"
	assert all(c_pressure.points == [1.0, 0.0])

The main task is determining how to implement testing without lots of fiddly mocking.