Document the Datasette(..., pdb=True) testing pattern

[simonw]

If you're writing tests for a Datasette plugin and you get a 500 error from inside Datasette, you can cause Datasette to open a PDB session within the application server code by doing this:

ds = Datasette([db_path], pdb=True)
response = await ds.client.get("/")

You'll need to run pytest -s to interact with the debugger, otherwise you'll get an error.

[simonw]

https://docs.datasette.io/en/latest/testing_plugins.html#using-pdb-for-errors-thrown-inside-datasette