Correcting Incorrect Usage of EfiSignatureList

[Flickdm]

An authenticated variable may contain only a single EfiSignatureList. However, they may contain multiple EfiSignatureLists otherwise known as EfiSignatureDatabase.

This change corrects that misconception

[Javagedes]

This looks fine, just needs to be updated to fix the unit tests

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.45%. Comparing base (ce85203) to head (74136e7).
Report is 56 commits behind head on master.

Files	Patch %	Lines
.../uefi/authenticated_variables_structure_support.py	83.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   81.43%   81.45%   +0.01%     
==========================================
  Files          56       56              
  Lines        7514     7542      +28     
==========================================
+ Hits         6119     6143      +24     
- Misses       1395     1399       +4     

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