Fix plugin tests not completely cleaning up after themselves

[djhoese]

We've had occasional test failures related to a missing "satpy_plugin". This seemed like a race condition by the pytest-xdist parallel test execution and some test corrupting other tests. Now that I tracked it down I think it could have happened if regular pytest execution ordered the tests differently. Basically the plugin tests were cleaning up the entry points cache before tests but they should have been doing it after the tests. This PR fixes that.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.13%. Comparing base (14c9f38) to head (869e4fe).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3074      +/-   ##
==========================================
+ Coverage   96.11%   96.13%   +0.01%     
==========================================
  Files         383      383              
  Lines       55685    55688       +3     
==========================================
+ Hits        53523    53534      +11     
+ Misses       2162     2154       -8     

Flag	Coverage Δ
behaviourtests	3.88% <0.00%> (-0.01%)	⬇️
unittests	96.22% <100.00%> (+0.01%)	⬆️

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