assert: ensure .rejects() disallows sync throws

This updates `assert.rejects()` to disallow any errors that are thrown
synchronously from the given function. Previously, throwing an error
would cause the same behavior as returning a rejected Promise.

Fixes: https://github.com/nodejs/node/issues/19646
PR-URL: https://github.com/nodejs/node/pull/19650


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>