Clean up PyTest config, coverage generation, unit tests

[zaneselvans]

• Moved all unit tests into a hierarchy under tests/unit
• Removed extraneous pytest options in tox.ini being set in pytest.ini
• The unit and etl testenvs in tox.ini now both append to the coverager
  report.
• The CI testenv now runs coverage erase to start with a fresh slate
• Removed unused @pytest.mark.datapkg decorators / cli option
• PyTest will now show warnings (of which there are a few)

Closes #893

[codecov]

Codecov Report

Merging #896 (0d8f010) into sprint29 (2061995) will increase coverage by 0.56%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           sprint29     #896      +/-   ##
============================================
+ Coverage     77.38%   77.94%   +0.56%     
============================================
  Files            44       44              
  Lines          5721     5721              
============================================
+ Hits           4427     4459      +32     
+ Misses         1294     1262      -32     

Impacted Files	Coverage Δ
src/pudl/workspace/datastore.py	71.03% <0.00%> (+1.40%)	⬆️
src/pudl/analysis/timeseries_cleaning.py	88.33% <0.00%> (+2.64%)	⬆️
src/pudl/extract/excel.py	97.22% <0.00%> (+2.78%)	⬆️
src/pudl/workspace/resource_cache.py	84.11% <0.00%> (+13.08%)	⬆️

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