tests: net: dhcpv4_server: Add test case for ICMP probing

Add test case which verifies that ICMP probing work as expected for the
DHCPv4 server. Make sure we build tests with both, probing enabled
(default) and disabled (enforced in testcase.yml) to make sure the
server is functional regardless of the probing feature.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>