[tests] document coverage option

bin/test.sh

@@ -75,7 +75,8 @@ function Usage
     echo "  -u|--update         run tests and update oracles (and WP-cache)"
     echo "  -s|--save           save dune logs into $DUNE_LOG"
     echo "  -v|--verbose        print executed commands"
-    echo "  -j|--jobs <jobs>    Run no more than <jobs> commands simultaneously."
+    echo "  -j|--jobs <jobs>    run no more than <jobs> commands simultaneously."
+    echo "  --coverage          compute test coverage"
     echo "  -h|--help           print this help"
     echo ""
     echo "VARIABLES"