[florimondmanca]

Following up comments from #614, this PR updates the tests so that:

• They prefer passing the string-form backend ("auto", "asyncio", "trio") to all pieces of public API.
• The "auto" option is put under test as well.

Fixes a bunch of NotImplementedErrors in the concurrency test utils as well.

Tests that use the backend are now twice as many (because the backend fixture now has two more params - auto-in-asyncio and auto-in-trio). I figured the test suite was still fast enough that running all these tests for each situation was acceptable, rather than e.g. unit-testing that passing "asyncio" looks up AsyncioBackend. Now that I wrote about it I'm less sure this wouldn't be preferable.

[florimondmanca]

Okay, I've moved things around a bit more so that:

• We only have two params for backend: auto-in-asyncio, auto-in-trio. (In particular this allows running tests for only one of the async libraries using pytest -m [asyncio|trio], which we'd lose with the previous pass.)
• We rely on auto-detection, as most users should, i.e. don't pass backend=backend to clients, connection pools or connections.
• We test passing the backend explicitly in a dedicated test.

[lovelydinosaur]

Looks ace, yup!

[StephenBrown2]

Can also remove the backend fixture param from all these test functions since it's not being referenced anywhere.

[florimondmanca]

Thanks, but actually we can’t — even though we don’t use the backend fixture in the test it’s responsible for setting up the asyncio/trio environment. I agree the usage as a function parameter is misleading though. We can probably switch to pytest.mark.usefixtures.

[florimondmanca]

We can probably switch to pytest.mark.usefixtures.

... and rename the fixture to « async_environment » or something? It’s true that the fixture doesn’t even return a backend class — it’s always « auto ». Hmm. Which means we might then get away with a custom mark, eg pytest.mark.is_async?

[StephenBrown2]

Yeah, I only suggested it since it is supposed to be auto-detected now, and they could be parameterized like https://github.com/encode/httpx/pull/615/files#diff-21bb0534f2645b17f8572db206999d00R165-R171
Which I now see is done here: https://github.com/encode/httpx/pull/615/files#diff-484462fced51d1a06b1d93b4a44dd535R52-R64 so, well done! Sorry I missed that before.
It is a little confusing as a function param, so I would agree with renaming it "async_backend" and then using it as @pytest.mark.usefixtures("async_backend") since all it's doing is adding the mark for asyncio/trio to do their thing.