tests: rtpaux: fix test failure

The RTP PT for alaw is 8.
Less than 50 packets are received in the length of this test so it
would never drop a buffer or would drop only the last buffer and
it would fail sometimes when the received wouldn't receive the
retransmission packet in time.

https://bugzilla.gnome.org/show_bug.cgi?id=746445
1 file changed