Add documentation and tests for returns

[HKuz]

Addresses #124

• Documentation: adds a screen shot and write-up for returns against purchase invoices in a Check Run
• tests

[agritheory]

@HKuz There's a test suit branch which has some of the strategies for this

[HKuz]

@agritheory - good call. I ended up copying over the conftest.py file and used your tests as a base. I had to make a few tweaks to get them to pass - namely splitting up the error test and the functionality tests for both the on-hold invoice and the returns (the CR wouldn't persist after the error test), then I had to make sure only a draft CR was returned in the fixture since we're using it more than once now.

This passes when I run pytest locally but we'll need the CI changes plus the recent v15 test workflow updates for it to pass CI.

[agritheory]

@HKuz I think this PR is the right one to remove the cypress tests.

[HKuz]

@agritheory - yup, makes sense to have the CI actually run the new tests... All set and passing 👍