🐛 Stream test bug fix

[prashantgupta24]

What this PR does / why we need it:

This PR adds:

• A fixture for the fastapi.testclient
• Skips testing streaming cases with FastAPI's testclient, pending resolution of RuntimeError during unit tests: Future attached to a different loop tiangolo/fastapi#10518
• Instead, test streaming tests against an actual server.

fix #540

Special notes for your reviewer:

• Following suggestions from:
• RunTimeError: got Future <Future pending> attached to a different loop when using custom loop in sync fixtures when upgrading from 0.14.2 to 0.15.0 encode/starlette#1315 (comment)
• RuntimeError during unit tests: Future attached to a different loop sysid/sse-starlette#68 (comment)

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility

[joerunde]

... so that it's used within a context manager.

But all the tests shown already were using it within a context manager? 😕😕😕

It looks like what this is doing is really moving to a single TestClient instead of having one per test. I'm totally down for this anyway since it's much DRYer but still confused about the root cause here.

[prashantgupta24]

@joerunde yeah the only difference was the session scoping of the fixture that was making it work. It doesn't work without that unfortunately :(

[joerunde]

Honestly, I'm here for the test code simplification, the fixture for the client is really nice. Let us know if you find out any more details about why this thing hangs!

[prashantgupta24]

@joerunde Now I'm thinking making the fixture session-scoped is not the right thing to do. Adding another test that pings an actual running server and using that along with either a function scoped or a session scoped test fixture doesn't seem to work :(

My gut feeling is that TestClient and EventSourceResponse don't seem to work well together. If I don't use TestClient, then running any number of tests against the stream endpoint seem to work.