Bluetooth: Mesh: Test pb cancelling in bsim

Adds a provision test to check that provisioning bearers are cancelled
when a provisioning link is opened. Modifies the functionality for
receiving unprovisioned beacons to fail the test if a unprovisioned
beacon is received from the current provisionee after a threshold.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>