Register test groups, mitigate warning

[ankona]

The pytest groups used to split execution of the test cases for parallelization currently result in a warning message displayed during test runs.

=== warnings summary ===
tests/test_cli.py:49
  /lus/cls01029/mcbridch/ss/tests/test_cli.py:49: PytestUnknownMarkWarning: Unknown pytest.mark.group_a - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    pytestmark = pytest.mark.group_a

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=== 52 passed, 1 warning in 0.53s ===

This fix adds group registrations to pyproject.toml to avoid those warnings:

=== 52 passed in 0.48s ===

[codecov]

Codecov Report

Merging #424 (c9407fb) into develop (15ba145) will decrease coverage by 0.06%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #424      +/-   ##
===========================================
- Coverage    89.65%   89.60%   -0.06%     
===========================================
  Files           59       59              
  Lines         3617     3617              
===========================================
- Hits          3243     3241       -2     
- Misses         374      376       +2     

see 1 file with indirect coverage changes

[billschereriii]

Thank you for catching and fixing this oversight!