tests: Bluetooth: ISO: BIS: Use controller buffer size as SDU size

Modify the test to use the controller buffer size as the maximum
SDU size, so per default we can send up to the maximum size
the controller supports. This ensures that the tests will
never do fragmentation over HCI.

This also adds verification of the received data on the
ISO sync receiver.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>