tests: net: trickle: Adjust config options to avoid error prints

Noticed this error print when running in mps2_an385 platform.

   <err> net_if: There is no network interface to work with!

Adding CONFIG_NET_LOOPBACK=y removes this error. After this
the CONFIG_NET_IPV6_MLD=n needed to be added in order to avoid
crashes when IPv6 MLD is done which is not needed for this test.

Removed also the whitelisting as that is not really needed and
we want to run these in as many platforms as possible.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>