Bluetooth: Controller: Fix regression in connection update

The connection event at the connection update instant was
skipped due to previous ticker node in use that is being
stopped was registering a relative occupied time that
overlapped with the new ticker node being started for
scheduling the connection with new interval.

Added mock interface for ticker_stop_abs() hence needed by
the Controller unit testing.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>