Bluetooth: audio: BAP broadcast assistant magic numbers and minor fixes

There are some hardcoded numbers in the test, which here are replaced
with the proper defines.

The test will now also FAIL if we go in an inapropriate state

A guard has been added around the call to bt_le_per_adv_sync_transfer,
so that it only gets called when PAST support is available

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>