tests/fate-run: Always overwrite output files for md5 tests

Otherwise the result of such tests will not accurately reflect the
current state.

Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

tests/fate-run.sh

@@ -158,7 +158,7 @@ md5pipe(){
 md5(){
     encfile="${outdir}/${test}.out"
     cleanfiles="$cleanfiles $encfile"
-    ffmpeg "$@" $(target_path $encfile)
+    ffmpeg -y "$@" $(target_path $encfile)
     do_md5sum $encfile | awk '{print $1}'
 }