Add test for checksum consistency

[zangell44]

Summary

This PR adds a unit test to ensure blocks schema checksums stay consistent for all Prefect registered blocks. This is a temporary measure to ensure no breaking changes are made while we work on a better versioning story.

Steps Taken to QA Changes

Checklist

This pull request is:

• [ ] A documentation / typographical error fix
  • No tests or issue needed
• [x] A short code fix
  • Please reference the related issue by including "closes <link to issue>" in this Pull Request's summary section.
    • If no issue exists, please create a bug report issue
  • Please include tests. One-line fixes without tests will not be accepted unless it's related to the documentation only.
• [ ] A new feature implementation
  • Please reference the related issue by including "closes <link to issue>" in this Pull Request's summary section.
    • If no issue exists, please create a feature enhancement issue
  • Please include tests
  • Please make sure that your QA steps are both thorough and easy to reproduce by somebody with limited knowledge of the feature that you are submitting

Happy engineering!