Add documentation for writing tests with signed actor cookies #1830
Description
I use this pattern in a lot of plugin tests, e.g. https://github.com/simonw/datasette-edit-templates/blob/087f6a6cabc20020f2b0524f11aa3a7836320848/tests/test_edit_templates.py#L55-L58
actor = ds.sign({"a": {"id": "root"}}, "actor")
response1 = await ds.client.get(
"/-/edit-templates/_footer.html", cookies={"ds_actor": actor}
)I should add this to the documentation on this page: https://docs.datasette.io/en/latest/testing_plugins.html