tests: carp: Update test case unicast_v4 to catch PR 284872

In the current test case unicast_v4, host one acts as a router, but it
uses if_bridge(4) to verify the reachability to CARP master.
Unfortunately, if_bridge(4) brings the two CARP hosts into the same
broadcast domain, thus with an unusual (undocumented) setup, one host
is in unicast mode but another one in multicast, or two hosts are both
in multicast mode, the test case can falsely report success.

Fix that by propagating route via routed(8).

PR 284872 shows that CARP(4) happens to send packets with multicast
destination when the unicast peer address is in the range x.x.x.224 ~
x.x.x.239. Update the peer address in test case to catch that bug.

PR: 284872
Reviewed by: kp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D49076

(cherry picked from commit 93fbdef51a13b8facac3633f55804a59ca27f635)