When we test backwards compatibility we often end up in a situation
where we *sometimes* get a warning, and sometimes don't. Like, we won't
get the warning if we're testing against an older version, but we will
in a newer one. Or we won't get the warning if the request randomly
lands on a node with an old version of the code. But we wouldn't if it
randomed into a node with newer code.
This adds `allowed_warnings` to our yaml test runner for those cases:
warnings declared this way are "allowed" but not "required".
Blocks #52959
Co-authored-by: Benjamin Trent <ben.w.trent@gmail.com>