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