[backport#19264] [tests] Don't import asyncio to test magic bytes

Summary:
49236be099c5e8b3cadbc98d5216313e7e1a5a45 [tests] Don't import asyncio to test magic bytes (John Newbery)

Pull request description:

Simplify the test for invalid start bytes. No need to import asyncio and the Network thread.

Backport of core#19264

Depends on D9519

Test Plan:

ninja all && ./test/functional/test_runner.py p2p_invalid_messages

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9520