FATE: specify the input format when decoding in enc_dec_pcm()

The output format is not always the same as the file extension,
which is sometimes required for correct probing. We can avoid
probing by specifying the format since it is already known.

tests/fate-run.sh

@@ -91,7 +91,7 @@ enc_dec_pcm(){
     encfile="${outdir}/${test}.${out_fmt}"
     cleanfiles=$encfile
     avconv -i $ref "$@" -f $out_fmt -y ${target_path}/${encfile} || return
-    avconv -i ${target_path}/${encfile} -c:a pcm_${pcm_fmt} -f wav -
+    avconv -f $out_fmt -i ${target_path}/${encfile} -c:a pcm_${pcm_fmt} -f wav -
 }
 
 regtest(){