test: Adjust delay for cam-app-example spawn

In the test "test_cam_app_multiple_application", the two
cam-app-example are meant to run concurrently, but since the
delay_s argument is not passed, the applications are started
with a default time offset. Correct this behavior passing a
zero delay.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>