dhclient/tests: use require.kmods property instead of ad-hoc checks

[lwhsu]

I'm good for kmods part. For dhcpd, in general it's a good direction. It's a bit pity that we lost the message of needing "ISC dhcp server", as I am not sure if this test works with other dhcpd. Not sure where is the best place to put that information.

[siva]

Added a comment pointing to the pkg origin of dhcpd

[siva]

Updated from discussion offline. I will commit this version instead. I will figure out a way to add "skip reason" metadata to Kyua in a future patch.