unit/test-gatt: Use auto security
unit/test-gatt: Use auto security

If the test don't expect any errors to happen use set security to auto so
bt_att will take care of elevating the security whenever needed.

pstglia/external-bluetooth-bluez,silent-snowman/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,ComputeCycles/bluez,silent-snowman/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pkarasev3/bluez,ComputeCycles/bluez