Add --show-program-output option to shell tests.

When used --show-program-output shows the output from the programs run
during testing.

Change-Id: I15a47c43d1fcf0243c8df1a75d0d2a584ae1f08f

test/decode_with_drops.sh

@@ -36,7 +36,7 @@ decode_with_drops() {
 
   [ -x "${decoder}" ] || return 1
 
-  "${decoder}" "${input_file}" "${output_file}" "${drop_mode}" > /dev/null 2>&1
+  eval "${decoder}" "${input_file}" "${output_file}" "${drop_mode}" ${devnull}
 
   [ -e "${output_file}" ] || return 1
 }