Makes pre/post test updates on schema and model

[akolson]

Summary

This pr adds a new constant for the preposttest to the mastery_model type and also updates the mastery_criteria schema

Note: For purposes of code readability preposttest has been updated to pre_post_test.

References

Closes #188

Reviewer guidance

• Ensure current schema tests pass and add test schema validations for the new "preposttest" type to confirm valid and invalid cases.
• Updates should be accessible in product eco-system (Kolibri, Studio, KDP, etc)

[LianaHarris360]

Thanks @akolson these changes are looking good! I just had a non-blocking question and minor note to add after looking over everything.