fate: add 'null' comparison method

Setting CMP=null uses stdout from the test as the diff record in
the report without further inspection.  This can be useful when
the exit status of the test itself determines success while the
output is informative.
Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/fate-run.sh

@@ -123,6 +123,7 @@ if test -e "$ref"; then
         diff)   diff -u -w "$ref" "$outfile"            >$cmpfile ;;
         oneoff) oneoff     "$ref" "$outfile" "$fuzz"    >$cmpfile ;;
         stddev) stddev     "$ref" "$outfile" "$fuzz"    >$cmpfile ;;
+        null)   cat               "$outfile"            >$cmpfile ;;
     esac
     cmperr=$?
     test $err = 0 && err=$cmperr