drop --color=always, it doesn't exist outside GNU world

tests/run-tests.sh

@@ -85,7 +85,7 @@ while read -r number test_line; do
     eval "curl -s $CHTSH_URL/$test_line" > "$TMP"
   fi
 
-  if ! diff -u3 --color=always results/"$number" "$TMP" > "$TMP2"; then
+  if ! diff -u3 results/"$number" "$TMP" > "$TMP2"; then
     if [[ $update_tests_results = NO ]]; then
       if [ "$show_details" = YES ]; then
         cat "$TMP2"