Fix plots in test scripts

[joeyshuttleworth]

Description

test_subtraction_plot.py now outputs an example subtraction plot. Fixed test_leak_correct.py plotting where the manually set times for the leak ramp were specified incorrectly -- now the test detects the right start and end times automatically.

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).