Only relying on the number of peers for detecting a new connection
suffers from race conditions, as unrelated previous peers could
disconnect at anytime in-between. Use the more robust approach of
watching for an increased highest peer id instead (again using the
`getpeerinfo` RPC call), with a newly introduced context manager
method `TestNode.wait_for_new_peer()`.
Fixes#29009.
Two recently added tests (PR #28625 / commit 2e31250027
and PR #28634 / commit 3bb51c29df)
introduced a bug by wrongly using the `assert_debug_log` helper.
Instead of passing the expected debug string in a list as expected, it
was passed as bare string, which is then interpretered as a list of
characters, very likely leading the debug log assertion pass even if the
intended message is not appearing.
In order to avoid bugs like this in the future, enforce that the
`{un}expected_msgs` parameters are lists.
