tests: Bluetooth: df: Add missing conn handle assignment

There was missing assignment to a global variable that stores
connection handle. The value was always the same, hence it test
were not failing. The value was passed over from setup state
done for other tests. This change makes sure there is always
correct handle stored in global variable.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>