test: lwm2m: Test cancellation using observe parameter

Implement test cases:
LightweightM2M-1.1-int-303 - Cancel observations using Observe with
Cancel parameter

LightweightM2M-1.1-int-305 - Cancel Observation-Composite Operation

Modify existing Leshan API to passive_cancel().

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>