Test framework

[alexcjohnson]

To run these tests locally, just make sure you have dash installed for testing - you can run pip install -r tests/requirements.txt - then just pytest to run all the tests, or pytest -k cd001 will just run the one test with cd001 in its function name. pytest -k cd001 --pause will run that same single test, but drop into pdb right after opening the browser window, so you can debug the test both in python (n a few times and you'll be in the test code) and in the browser (open the js console and do whatever you like)

I added two example tests - one showing dragging & pinning columns and verifying that they're drawn as expected after, another filtering from the ui. I didn't add any tests with dash callbacks, but those work the same way. Both tests use the utils.Grid class to centralize all the ag-grid-specific selectors.

[alexcjohnson]

I had to downgrade to node 16 - pretty sure this is fixed in recent versions of Webpack (see this SO post), but this project is still on v4 so I'll upgrade to v5 in a separate PR and see if that lets us upgrade to node 18