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

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Backport of [[https://github.com/bitcoin/bitcoin/pull/19264 | 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

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