tests: net: Fix assignments to CONFIG_NET_DEBUG_6LO

This symbol was renamed by commit 14e73e73 ("net: Make DEBUG config
option naming consistent"), but an assignment (and a commented-out
assignment) to the old name remained.

Fix the assignments to use the new name.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>