feat(bindings): add node, python, swift tests

[ObserverOfTime]

The python bindings currently segfault instead of raising an error so that needs to be fixed first.

Test commands:

node --test bindings/node/*_test.js
python -m unittest discover bindings/python/tests
swift test

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For what it's worth, these tests on their own are not particulary useful,
but they're there so that grammar authors can easily add more if they're interested.