Fix flaky hls keepalive test (#47186)

Remove a call to stream.start() which is issued before the test is fully
setup (e.g. keepalive is not set to True, and mock calls are not registered)

tests/components/stream/test_hls.py

@@ -229,7 +229,6 @@ async def test_stream_keepalive(hass):
     stream = create_stream(hass, source)
     track = stream.add_provider("hls")
     track.num_segments = 2
-    stream.start()
 
     cur_time = 0