tests: Bluetooth: bsim_test_iso: Change wait time in BIG receiver

The receiver and transmitter in the test are synchronized by
use of sleep functions. The change in handling of periodic
advertising synchronized event caused missmach in waiting
times. Receiver is notified about established synchronization
later that it was in the past. Due to that the wait for end
of transmitter operation was too long.

Temporary fix for the problem is decrease of receiver sleep
time by one periodic advertising interval.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>