Websockets implementation does not clean properly tasks if handshake fails

[euri10]

this is a good explanation of what we see in those 2 tests and that causes flaky tests:

FWIW there's an asyncio warning when I try to run test_send_before_handshake or test_missing_handshake in isolation. Doesn't show if I run either along with other tests, eg either the full test suite, with -k websocket, or the two tests together with -k "missing_handshake or before_handshake".

$ pytest -k test_missing_handshake
====================================================== test session starts =======================================================
platform darwin -- Python 3.9.0, pytest-6.1.1, py-1.9.0, pluggy-0.13.1
rootdir: /Users/florimond/Developer/python-projects/uvicorn, configfile: setup.cfg
plugins: mock-3.3.1, asyncio-0.14.0
collected 213 items / 211 deselected / 2 selected                                                                                

tests/protocols/test_websocket.py ..                                                                                       [100%]

=============================================== 2 passed, 211 deselected in 2.30s ================================================
Task was destroyed but it is pending!
task: <Task pending name='Task-11' coro=<WebSocketServerProtocol.handler() done, defined at /Users/florimond/Developer/python-projects/uvicorn/venv/lib/python3.9/site-packages/websockets/server.py:118> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x10a7e1760>()]>>

Originally posted by @florimondmanca in #918 (comment)