Add test for nc.LinearBiasSystematic

[marcpaterno]

Description

This PR introduces additional testing to cover previously untested code.

Type of change

Please delete the bullet items below that do not apply to this pull request.

• Refactoring

Checklist:

The following checklist will make sure that you are following the code style and
guidelines of the project as described in the
contributing page.

• I have run bash pre-commit-check and fixed any issues
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have 100% test coverage for my changes (please check this after the CI system has verified the coverage)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (1d8494a) to head (294f0fc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #467     +/-   ##
========================================
+ Coverage    99.0%   99.2%   +0.1%     
========================================
  Files          54      54             
  Lines        4147    4149      +2     
========================================
+ Hits         4107    4117     +10     
+ Misses         40      32      -8     

Files with missing lines	Coverage Δ
firecrown/parameters.py	100.0% <100.0%> (+0.6%)	⬆️

... and 1 file with indirect coverage changes