fate/ffmpeg: remove comparison against ref from fix_sub_duration_heartbeat

After the full ffmpeg CLI multithreading changes went in, this
test started depending on how far the input side read and decoded
the input compared to how quickly the output encoded things, causing
spurious failures on the CI.

To my knowledge all of the failures have so far been valid correct
results, but unfortunately FATE's built in checks mostly consist of
whether there is a difference against an exact result.

This way we still get the CI and valgrind running of the code,
but stop its comparison. Reference file is left around so that
the previous reference is still available.

tests/fate/ffmpeg.mak

@@ -133,6 +133,11 @@ fate-ffmpeg-fix_sub_duration_heartbeat: CMD = fmtstdout srt -fix_sub_duration \
   -c:v mpeg2video -b:v 2M -g 30 -sc_threshold 1000000000 \
   -c:s srt \
   -f null -
+# FIXME: disabling comparison against reference as after ffmpeg multithreading
+#        went in, this test started depending on how far the input side
+#        progressed compared to how quickly the output encoded packets,
+#        causing spurious failures on the CI.
+fate-ffmpeg-fix_sub_duration_heartbeat: CMP = null
 
 # FIXME: the integer AAC decoder does not produce the same output on all platforms
 # so until that is fixed we use the volume filter to silence the data