Migrate to pytest: mongodb

[thinkst-pieter]

Proposed changes

The firs of a couple of PR's to migrate the testing to pytest from the python unit test into a dedicated test file for each opencanary module in preparation of adding more testing.

This PR handles the mongodb tests

Types of changes

• Bugfix (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 not work as expected)
• Documentation Update

Checklist

• Lint and unit tests pass locally with my changes (if applicable)
• I have run pre-commit (pre-commit in the repo)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Linked to the relevant github issue or github discussion