tests: Bluetooth: df: Add missing intergration_platforms

Add intergration_plaforms entry in testcase.yaml.
Thanks to that nrf52_bsim platform will be used instead
of default platforms that do not include nrf52_bsim.
The test will be not dead as it is now (executed only
when tests code has been changed).

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