Refactor tests into utility unit tests and bigger unit tests

[dandhlee]

Googlers see b/206698957.

For functions that are small utility functions, they should be in a separate unit test file since tests/test_unit.py is getting rather large. Will keep using tests/test_unit.py for bigger tests for stuff like checking docstring parsing, but not for utility functions like find_package_group.

[dandhlee]

The test filename besides test_unit can be confusing. Let's be careful on naming and not confuse other contributors/reviewers.

Suggestion given: keeping it all in one file or only breaking up by class/type/functionality being tested.

[dandhlee]

In tox.ini file, instead of using python3 -m unittest tests/test_unit.py tests/test_helpers.py tests/test_markdown.py to list out all the files, try https://docs.python.org/3/library/unittest.html#test-discovery.