fate: explicitly specify audio bitrate for adpcm/mp2fixed tests.

They picked up defaults, which changes from 128 to 200 when we remove
FF_API_OLD_AVOPTIONS.

tests/fate/acodec.mak

 fate-acodec-%: CODEC = $(@:fate-acodec-%=%)
 fate-acodec-%: SRC = tests/data/asynth-44100-2.wav
-fate-acodec-%: CMD = enc_dec wav $(SRC) $(FMT) "-b 128k -c $(CODEC) $(ENCOPTS)" wav "-c pcm_s16le $(DECOPTS)" -keep
+fate-acodec-%: CMD = enc_dec wav $(SRC) $(FMT) "-b:a 128k -c $(CODEC) $(ENCOPTS)" wav "-c pcm_s16le $(DECOPTS)" -keep
 fate-acodec-%: CMP_UNIT = 2
 fate-acodec-%: REF = $(SRC_PATH)/tests/ref/acodec/$(@:fate-acodec-%=%)
 
@@ -93,6 +93,7 @@ fate-acodec-mp2: ENCOPTS = -b:a 128k
 FATE_ACODEC-$(call ENCDEC, MP2FIXED MP2 , MP2 MP3) += fate-acodec-mp2fixed
 fate-acodec-mp2fixed: FMT = mp2
 fate-acodec-mp2fixed: CMP_SHIFT = -1924
+fate-acodec-mp2fixed: ENCOPTS = -b:a 384k
 
 FATE_ACODEC-$(call ENCDEC, ALAC, MOV) += fate-acodec-alac
 fate-acodec-alac: FMT = mov