Fix subtle naming differences between DummyCube & iris cubes

[truth-quark]

There seen to be some subtle differences between iris cubes and our DummyCube which would cause the test_fix_level_coord_... tests to fail if we were using real iris cubes.

The cube.coord() method uses the current name for a DimCoord as its argument, where the "current name" comes from either the DimCoord's standard_name, long_name, or var_name, with the following order of priority: standard_name > long_name > var_name.

This means that if a DimCoord only has a var_name attribute, changing the var_name forces us to also change the argument in cube.coord(...) used to access it:

A made up example after modifying a loaded cube's coordinate names:

>>> cube.coord("pseudo_level").__dict__
{'_metadata_manager': MetadataManager(standard_name=None, long_name=None, var_name='pseudo_level', units=Unit('1'), attributes={}, coord_system=None, climatological=False, circular=False), '_values_dm': DataManager(array([1, 2, 3, 4, 5, 6])), '_bounds_dm': None, '_ignore_axis': False}

>>> cube.coord("pseudo_level").var_name = "new_var_name"

>>> cube.coord("pseudo_level")

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/g/data/vk83/apps/payu/1.1.5/lib/python3.10/site-packages/iris/cube.py", line 2304, in coord
    raise iris.exceptions.CoordinateNotFoundError(emsg)
iris.exceptions.CoordinateNotFoundError: "Expected to find exactly 1 'pseudo_level' coordinate, but found none."

>>> cube.coord("new_var_name")
<DimCoord: new_var_name / (1)  [1, 2, ..., 5, 6]  shape(6,)>

In these tests however, we're changing the coordinate's var_names but not the cube.coord(...) arguments used to access them.

Given that the tests are working and it's a fairly subtly difference, should we defer this to a separate issue?

Originally posted by @blimlim in #113 (comment)