TST: Update test in #28798 to use a Categorical

[topper-123]

• xref BUG: Keep categorical name in groupby #28798
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

I merged #28798 too fast, as the column "Name" wasn't a Categorical, which was to be tested. I've made it a Categorical and for good measure I've added ordered_fixture to the test.

[topper-123]

I get this error, which doesn't seem to be related to my PR:

pytest -m "single and not slow and not network" -n 1 -s --strict --durations=10 --junitxml=test-data-single.xml pandas
Traceback (most recent call last):
  File "/home/vsts/miniconda3/envs/pandas-dev/bin/pytest", line 11, in <module>
    sys.exit(main())
  File "/home/vsts/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/config/__init__.py", line 58, in main
    config = _prepareconfig(args, plugins)
  File "/home/vsts/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/config/__init__.py", line 182, in _prepareconfig
    config = get_config()
  File "/home/vsts/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/config/__init__.py", line 153, in get_config
    pluginmanager.import_plugin(spec)
  File "/home/vsts/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/config/__init__.py", line 522, in import_plugin
    __import__(importspec)
  File "/home/vsts/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/tmpdir.py", line 25, in <module>
    class TempPathFactory(object):
  File "/home/vsts/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/_pytest/tmpdir.py", line 35, in TempPathFactory
    lambda p: Path(os.path.abspath(six.text_type(p)))
TypeError: attrib() got an unexpected keyword argument 'convert'
##[error]Bash exited with code '1'.

This doesn't seem to be related to this PR.