test_dhcp_agent: Fix no-op tests

Fix some uses of assertCalledOnceWith, which seems like a mistake of
assert_called_once_with.

Also, add a hacking check to prevent the mistake.

Closes-Bug: #1397184
Change-Id: I12d077e2724d52eff65d55aff1130fbbb69671b1
Merge "test_dhcp_agent: Fix no-op tests"