dhcp: T6948: Smoketest for DHCP server leases sync with hostd records

The test test_dhcp_hostsd_lease_sync validates
DHCP server leases in sync with hostd records.

Since the DHCP server running in smoketest environment
might not have actual clients to lease IP addresses,
we simulate the lease creation and deletion using kea
helper functions (kea_add_lease and kea_delete_lease).