BackendFixture fails with Sanic because missing `test_mode`

[rmorshea]

Discussed in #860

^{Originally posted by ahopkins December 29, 2022}
The problem was reported here: sanic-org/sanic#2643.

The issue is that the BackendFixture for testing Sanic does not set Sanic.test_mode = True, which means that there is potential for issues (as seen the linked issue) to arise. I propose that you add this into your test implementation for Sanic to automatically manage setting that variable.

[rmorshea]

I think this was fixed by: #862