Bluetooth: Mesh: Reduce number of reprovision runs in test

The pb_adv_reprovision test takes a long time to execute, as it's
running 10 provisioning sessions in a row. Any side effects from the
provisioning process should come into play already on the first
reprovisioning, so we can safely reduce this to only 3 provisioning runs
without losing any test coverage.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>