Prior to the recent shunit2 to Hatchet test conversion, there were
already a few Hatchet tests. This cleans those up slightly:
* Makes `ci_spec.rb` use a fixture in the repository, instead of a
  mixture of remote repository (`sharpstone/python_default`) and
  content written out at test runtime.
* Removes `sharpstone/python_default` from `hatchet.json` since there
  is nothing left using it.
* Renames `python_spec.rb` to `getting_started_spec.rb` since that's
  the only test left in that file.

Closes @W-8786787@.

[skip changelog]