sd-bus: add test cases for truncated fds

sd-bus: add test cases for truncated fds

We add some test cases for the previous commits: first (with Claude's
help) we exercise the message creation API internally by passing it
various combinations of incorrect fds with the might_be_truncated flag
set to true or false.

Then we try more of a "real world" test by lowering our fd limit and
sending ourselves a message via the bus and making sure that we
successfully receive a message that has had at least some of its fds
truncated.