test: Fix p2p_invalid_messages failing in Python 3.8 because of warning #17931

In Python 3.8 p2p_invalid_messages.py fails because of the following warning python produce:

2020-01-15T13:02:14.486000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_3xq0f6uh
./test/functional/p2p_invalid_messages.py:154: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
  asyncio.run_coroutine_threadsafe(asyncio.coroutine(swap_magic_bytes)(), NetworkThread.network_event_loop).result()
2020-01-15T13:02:15.306000Z TestFramework (INFO): Sending a bunch of large, junk messages to test memory exhaustion. May take a bit...
2020-01-15T13:02:17.971000Z TestFramework (INFO): Waiting for node to drop junk messages.
2020-01-15T13:02:18.042000Z TestFramework.mininode (WARNING): Connection lost to 127.0.0.1:12826 due to [Errno 104] Connection reset by peer
2020-01-15T13:02:18.141000Z TestFramework (INFO): Sending a message with incorrect size of 2
2020-01-15T13:02:18.293000Z TestFramework (INFO): Sending a message with incorrect size of 77
2020-01-15T13:02:18.344000Z TestFramework.mininode (WARNING): Connection lost to 127.0.0.1:12826 due to [Errno 104] Connection reset by peer
2020-01-15T13:02:18.445000Z TestFramework (INFO): Sending a message with incorrect size of 78
2020-01-15T13:02:18.597000Z TestFramework (INFO): Sending a message with incorrect size of 79
2020-01-15T13:02:18.902000Z TestFramework (INFO): Stopping nodes
2020-01-15T13:02:19.154000Z TestFramework (INFO): Cleaning up /tmp/bitcoin_func_test_3xq0f6uh on exit
2020-01-15T13:02:19.154000Z TestFramework (INFO): Tests successful

so as it says I replaced the co-routine with async def which IIUC is supported since Python 3.5, so this makes the test pass both on 3.5+ and on 3.8 https://docs.python.org/3.5/library/asyncio-task.html ("The async def type of coroutine was added in Python 3.5, and is recommended if there is no need to support older Python versions")

I think it's somewhat strange for the test to fail on a warning, but I also think this change makes sense.

ACK f117fb00da747147cddfb071c1427a2754c278cd if it passes travis

I think currently the tests fail if the python side prints anything to stdout/stderr

ACK f117fb00da747147cddfb071c1427a2754c278cd - observed the failure (it's the only test that fails) with Python 3.8.1, tested the fix with 3.5.6 and 3.8.1. This is our only usage of asyncio.coroutine.

72/136 - p2p_invalid_messages.py failed, Duration: 9 s

stdout:
2020-01-16T09:37:44.223000Z TestFramework (INFO): Initializing test directory /var/folders/z2/cn877pxd3czdfh47mfkmbwgm0000gn/T/test_runner_₿_🏃_20200116_173347/p2p_invalid_messages_62
2020-01-16T09:37:46.045000Z TestFramework (INFO): Sending a bunch of large, junk messages to test memory exhaustion. May take a bit...
2020-01-16T09:37:51.293000Z TestFramework (INFO): Waiting for node to drop junk messages.
2020-01-16T09:37:51.344000Z TestFramework (INFO): Skipping test p2p_invalid_messages/1 (oversized message) under macOS
2020-01-16T09:37:51.348000Z TestFramework (INFO): Sending a message with incorrect size of 2
2020-01-16T09:37:51.413000Z TestFramework.mininode (WARNING): Connection lost to 127.0.0.1:11744 due to [Errno 54] Connection reset by peer
2020-01-16T09:37:51.525000Z TestFramework (INFO): Sending a message with incorrect size of 77
2020-01-16T09:37:51.585000Z TestFramework.mininode (WARNING): Connection lost to 127.0.0.1:11744 due to [Errno 54] Connection reset by peer
2020-01-16T09:37:51.692000Z TestFramework (INFO): Sending a message with incorrect size of 78
2020-01-16T09:37:51.880000Z TestFramework (INFO): Sending a message with incorrect size of 79
2020-01-16T09:37:52.212000Z TestFramework (INFO): Stopping nodes
2020-01-16T09:37:52.691000Z TestFramework (INFO): Cleaning up /var/folders/z2/cn877pxd3czdfh47mfkmbwgm0000gn/T/test_runner_₿_🏃_20200116_173347/p2p_invalid_messages_62 on exit
2020-01-16T09:37:52.691000Z TestFramework (INFO): Tests successful


stderr:
/Users/michael/github/bitcoin/test/functional/p2p_invalid_messages.py:154: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
  asyncio.run_coroutine_threadsafe(asyncio.coroutine(swap_magic_bytes)(), NetworkThread.network_event_loop).result()