Add testing for subtraction plot function

[joeyshuttleworth]

Description

Fixes #11 by including a unit test that checks that do_subtraction_plot runs without errors. Change the parameters of do_subtraction_plot so that testing is easier.

Additionally, add a line to run run_herg_qc.py in the testing workflow

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically

Documentation checklist

• I have updated all documentation in the code where necessary.
• I have checked spelling in all (new) comments and documentation.
• I have added a note to RELEASE.md if relevant (new feature, breaking change, or notable bug fix).

[joeyshuttleworth]

Merged into #9