Adds missing __init__.py files [FC-0030]

[pomegranited]

Adds missing __init__.py files, and fixes the resulting linting errors, plus some minor test fixes:

• ensures 100% test coverage for files under tests/ (see previous coverage report)
• fixes PytestCollectionWarnings (see previous warnings)

This change shouldn't require any manual testing, just a code review.

Follow-up to #87 (comment)

Private-ref: FAL-3477

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[ChrisChV]

@pomegranited Looks good to me, thanks for this work 👍

• I tested this: A ran the tests and checked the coverage and warnings
• I read through the code and considered the security, stability and performance implications of the changes.
• Includes documentation.

[bradenmacdonald]

Thanks, very nice cleanup! Just one little thing that looks risky to me, and otherwise it's awesome and I'll approve and merge.

[bradenmacdonald]

Thanks, very nice work! Please fix the conflict and merge at your convenience.

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