Fix the test prohibiting jails from sharing IP addresses.

It's not supposed to be legal for two jails to contain the same IP address,
unless both jails contain only that one address. This is the behavior
documented in jail(8), and is there to prevent confusion when multiple
jails are listening on IADDR_ANY.

VIMAGE jails (now the default for GENERIC kernels) test this correctly,
but non-VIMAGE jails have been performing an incomplete test when nested
jails are used.

Approved by: re@ (kib@)
MFC after: 5 days