test_web_sendfile_functional isn't testing sendfile

[Dreamsorcerer]

We have this code in a fixture:

aiohttp/tests/test_web_sendfile_functional.py

Lines 75 to 76 in 8a525d9

	if request.param == "no_sendfile":
	asyncio.set_event_loop(loop_without_sendfile)

This looked rather dodgy as it appears, at first glance, to be changing the event loop in the middle of a handler, which you'd expect to break a lot of things.
However, in reality, the loop_without_sendfile fixture modifies the object from the loop fixture, which will be the default loop currently running. Which means that the loop has always been modified and this set_event_loop() call is a no-op.

That means the fixture is causing all the tests to be run twice under the exact same conditions. There is no test case where the sendfile has not been patched out. The fixtures need to be reworked to make this behave correctly, and a regression test for the fixture needs to be added.