Regression test for creating a rdomain default loopback interface.

Even if a lo(4) interface will now be created for every rdomain, we
must ensure that condiguring such interface via netstart(8) still
work, for example:

$ cat /etc/hostname.lo1
rdomain 1
127.0.0.1
inet6 ::1/128
alias 88.76.0.3/32

Add regress test on local route insertions