Allow 'glm_l2' reader to accept arbitrary filename prefixes

[djhoese]

Plans are coming down the pipe that will make it more common for arbitrary filename prefixes in PUG-like files. Specifically the CSPP Geo Gridded GLM package will default to a CSPP_CG prefix instead of the operational realtime prefix OR.

• Tests added

[ghost]

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[codecov]

Codecov Report

Merging #1563 (8bf3ef8) into master (21ebf26) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1563      +/-   ##
==========================================
+ Coverage   92.21%   92.54%   +0.33%     
==========================================
  Files         251      251              
  Lines       36761    36761              
==========================================
+ Hits        33900    34022     +122     
+ Misses       2861     2739     -122     

Flag	Coverage Δ
behaviourtests	4.49% <0.00%> (ø)
unittests	92.68% <100.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_glm_l2.py	100.00% <100.00%> (ø)
satpy/readers/glm_l2.py	98.55% <0.00%> (+2.89%)	⬆️
satpy/utils.py	81.95% <0.00%> (+58.53%)	⬆️

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