test: Rename send_message to send_without_ping
send_message is problematic, because it is easy to forget a sync_with_ping (or other wait_until), leading to intermittent test failures. (Example: https://github.com/bitcoin/bitcoin/pull/31837#discussion_r1950370246)
There are more uses of send_and_ping in the codebase than send_message, so in most cases send_and_ping is needed anyway.
For the remaining cases, clearly document that no sync happens by renaming send_message to send_without_ping.