Alternative, async-friendly pattern for make_app_client() and similar - fully retire TestClient

[simonw]

In this issue I replaced a whole bunch of places that used the non-async app_client fixture with an async ds_client fixture instead:

• Refactor test suite to use mostly async def tests #1959

But I didn't get everything, and a lot of tests are still using the old TestClient mechanism as a result.

The main work here is replacing all of the app_client_... fixtures which use variants on the default client - and changing the tests that call make_app_client() to do something else instead.

This requires some careful thought. I need to come up with a really nice pattern for creating variants on the ds_client default fixture - and do so in a way that minimizes the number of open files, refs:

• Intermittent "Too many open files" error running tests #1843

[simonw]

Also need an alternative mechanism to this convenience for getting CSRF tokens before a POST:

datasette/datasette/utils/testing.py

Lines 90 to 103 in 5ee954e

	cookies = cookies or {}
	post_data = post_data or {}
	assert not (post_data and body), "Provide one or other of body= or post_data="
	# Maybe fetch a csrftoken first
	if csrftoken_from is not None:
	assert body is None, "body= is not compatible with csrftoken_from="
	if csrftoken_from is True:
	csrftoken_from = path
	token_response = await self._request(csrftoken_from, cookies=cookies)
	csrftoken = token_response.cookies["ds_csrftoken"]
	cookies["ds_csrftoken"] = csrftoken
	post_data["csrftoken"] = csrftoken
	if post_data:
	body = urlencode(post_data, doseq=True)

One option would be adding that mechanism to datasette.client.post(...) - as a _csrf_from= parameter (with an underscore because it's mainly intended for use in tests, though perhaps that's a weird convention that I shouldn't introduce).