src/bin/sage: Show tox environment list in 'sage -advanced'

src/bin/sage

@@ -404,12 +404,14 @@ usage_advanced() {
     if [ -n "$SAGE_SRC" -a -f "$SAGE_SRC/tox.ini" ]; then
         echo "  --tox [options] <files|dirs> -- general entry point for testing"
         echo "                                  and linting of the Sage library"
-        echo "     --listenvs -v    -- show list of test environments"
-        echo "     -e <envlist>     -- run tests environments"
+        echo "     -e <envlist>     -- run specific test environments (default: run all)"
+        tox -c "$SAGE_SRC" --listenvs -v 2>/dev/null | sed -n '/->/s/^/        /;s/(same as/\
+                            (same as/;s/->/     --/p;'
+        echo "     -p auto          -- run test environments in parallel"
         echo "     --help           -- show tox help"
+        command -v tox &>/dev/null || \
+        echo "                         (not installed currently, run sage -i tox)"
     fi
-    command -v tox &>/dev/null || \
-    echo "                         (not installed currently, run sage -i tox)"
     echo
     echo "Some developer utilities:"
     echo