ipsec: T1210: extend remote-access smoketest with IP pool configuration

This extends commit rVYOSONEXf9207ed4a0a5 ("ipsec: T1210: add smoketest for remote-access
(road-warrior) users") in a way that also the IPv4 pool and its DNS servers get
validated. There is no separate IPv6 test, as both address families behave
the same way when configuring these.

(cherry picked from commit 1e46cd606d9d87226fe0400bf3a53bda360808d8)