Modularize testing suite

[tylerhutcherson]

This PR introduces a few improvements to our test suite to make it more modular.

1. Adds the ability to run tests against specific file(s) using the poetry scripting:

poetry run test tests/unit/test_fields.py

2. Adds the ability to toggle the API-enabled tests via command line arg using the poetry scripting

poetry run test-verbose --run-api-tests

OR

make test-all

This removes the need to set those ugly SKIP_VECTORIZER=true flags.

And run without the API dependencies:

poetry run test-verbose

make test

3. Wraps notebook tests into a convenient poetry script and makefile wrapper:

poetry run test-notebooks

OR

make test-notebooks

[abrookins]

This is nice because you can pass -n 0 to disable xdist and it'll override the earlier -n arg.

[abrookins]

Couple of comments but LGTM!

[rbs333]

LGTM

[bsbodden]

Nice improvements!