* Where does pathlib.Path('tests/data') resolve the path from? It's
  relative to the current working directory.
* Where is the tests/data directory? Relative to the tests.
* PyCharm has quirks related to the current working directory when
  running and debugging tests.
* By using __file__ to resolve the test data directory, we remove the
  non-guaranteed assumption that the current working directory is always
  at the root of the repository, and we replace that assumption with a
  value that is always correct regardless of current working directory.
* As a result, the unit tests can now be debugged in PyCharm, unit tests
  can be executed from any directory, and test execution continues to
  work in all the places it originally worked.

Some of the handling is slightly messy but that can be cleaned up by
making the task config files themselves not dependent on the CWD

Signed-off-by: d10n <[email protected]>