netinet tests: Add error handling tests for UDP with v4-mapped sockets

This provides a regression test for commit abca3ae7734f.

Add it to the existing v4-mapped address test file, and rename
accordingly.

Reviewed by: tuexen, karels, rrs
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39216