Refactor tests to use MockTransport(<handler_function>)

[tomchristie]

Refactors our test cases so that we're able to test against simple request/response handler functions.

import httpx
from tests.utils import MockTransport

def app(request: httpx.Request) -> httpx.Response:
    ...

def test_something():
    client = httpx.Client(transport=MockTransport(app))
    response = client.get(...)

This is actually kind of a big deal. 🤗

Everything is still passing through our really tightly defined transport API, but we get the benefits of being able to work with proper request/response instances, rather than having to build out test behaviours by scratching around with bytewise variables, raw header lists, and httpcore.SyncIteratorStream/httpcore.AsyncIteratorStream instances.

Still need to clean up client/test_auth.py using this, but have covered all the other client test modules.

The implementation of MockTransport in the tests/utils.py module also gives us a really good concrete case to work against for working through some of the more nitty gritty details of the request/response APIs.

Really we'd like for the implementation to be as simple as...

class MockTransport(httpcore.SyncHTTPTransport):
    def __init__(self, handler: Callable) -> None:
        self.handler = handler

    def request(
        self,
        method: bytes,
        url: Tuple[bytes, bytes, Optional[int], bytes],
        headers: List[Tuple[bytes, bytes]] = None,
        stream: httpcore.SyncByteStream = None,
        timeout: Mapping[str, Optional[float]] = None,
    ) -> Tuple[bytes, int, bytes, List[Tuple[bytes, bytes]], httpcore.SyncByteStream]:
        request = httpx.Request(method=method, url=url, headers=headers, content=stream)
        request.read()
        response = self.handler(request)
        return (
            response.http_version.encode("ascii"),
            response.status_code,
            response.reason_phrase.encode("ascii"),
            response.headers.raw,
            response.stream
        )

(In fact in addition to that, we'd probably also like to end up with an API that allows MockTransport be usable with either sync or async clients, rather than having to have separate MockTransport and AsyncMockTransport classes, but let's save that conversation for another day.)

The existing implementation here gives us something to work against, while tackling little parts of that incrementally. Eg. adding support for passing raw URL tuples of (<bytes>, <bytes>, <optional int>, <bytes>) to httpx.URL(...), and also some of the details around #1253

Related to #1091

[tomchristie]

It's a bit fiddly to explain why we're having to do this at the moment, but it's related to some of the grungy bits that need ironing out in #1253.

I'll resolve it as part of a follow-up.

[florimondmanca]

Very nice indeed! 💯 👍