tests: net: ieee802154: Set LL addresses on a packet

The reworked 6lowpan implementation asserts when no LL source or
destination address is set on a packet. This caused the fragmentation
tests to fail before they completed.

Fix the issue, by setting a dummy address on a packet during a setup.

Fixes #19761

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>