mgcp: Use l16 in the test

G729 might not be available, so execute the test with codecs that
are always available.
diff --git a/openbsc/tests/mgcp/mgcp_transcoding_test.ok b/openbsc/tests/mgcp/mgcp_transcoding_test.ok
index 5cfc289..387cfd2 100644
--- a/openbsc/tests/mgcp/mgcp_transcoding_test.ok
+++ b/openbsc/tests/mgcp/mgcp_transcoding_test.ok
@@ -536,4 +536,4 @@
 generating 160 pcma input samples
 got 1 pcma output frames (80 octets) count=12
 got 1 pcma output frames (80 octets) count=12
-Testing Initial G729->GSM, PCMA->GSM
+Testing Initial L16->GSM, PCMA->GSM