tests: net: conn_mgr: Use valid LL address in tests

Using L2 address of length 1 (invalid/unsupported one) confused IPv6
layer during LL address generation - since that length was not a valid
one, the address was not initialized properly and a part of it was set
semi-random. This could result for example in filling out the neighbor
tables.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 3089a5d1)