Pytest CLI option for log-level

[reuterbal]

As discussed offline: This provides a CLI option overwrite for the log-level when running pytest, and removes the duplicate env variable hook from the config.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/396/index.html

[mlange05]

Yes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.52%. Comparing base (44df609) to head (3e0a5bf).
Report is 3 commits behind head on naml-loglevel-detail.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           naml-loglevel-detail     #396   +/-   ##
=====================================================
  Coverage                 95.52%   95.52%           
=====================================================
  Files                       186      186           
  Lines                     39320    39320           
=====================================================
  Hits                      37562    37562           
  Misses                     1758     1758           

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	95.51% <100.00%> (ø)

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