Add documentation for writing tests with signed actor cookies

[simonw]

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

[simonw]

This can now be done with datasette.client.actor_cookie({"id": "root"}).

datasette/datasette/app.py

Lines 1884 to 1891 in 48148e6

	class DatasetteClient:
	def __init__(self, ds):
	self.ds = ds
	self.app = ds.app()
	def actor_cookie(self, actor):
	# Utility method, mainly for tests
	return self.ds.sign({"a": actor}, "actor")

[simonw]

New docs are here: https://docs.datasette.io/en/latest/testing_plugins.html#using-datasette-client-in-tests