tests: Bluetooth: BAP: Make RX checks less strict

Modify stream_rx.c so that the calls to FAIL are guarded
by the newly added valid_rx_cnt.

This helps prevent timing issues, especially with
broadcast, where the first SDU(s) may be delayed from
the application, and thus may be missed/contain errors.
Now it will only treat missing or error SDUs as a FAIL
if we have received a valid SDU. All fails, will still be
logged. We now also log both valid and total count.

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