testing: Only run DHCPv4 by setting an listening interface explicitly

testing: Only run DHCPv4 by setting an listening interface explicitly

Debian stretch's init script for isc-dhcp-server uses the INTERFACESv4|6
variables to decide whether to start the v4 and/or v6 DHCP server.

If they are not empty, the daemon is started for the respective version,
however, if both are empty (the default), to listen on all interfaces, the
daemon is started for both versions.  The latter would require a subnet
config for IPv6 as the daemon otherwise exits, letting the init script fail,
while keeping the successfully started v4 version running, which, in turn,
can't be stopped anymore with the init script because it thinks the daemon
is not running.

So it's not possible with this init script to start DHCPv4 on all interfaces
without having to configure and run DHCPv6 also.