Added list.reverse() unittest

test/unittest/list_reverse.gravity

@@ -0,0 +1,10 @@
+#unittest {
+    name: "List reverse.";
+    result: "[4,2,1,6,4]";
+};
+
+func main() {
+  var list = [4, 6, 1, 2, 4]
+  list.reverse()
+  return list.String()
+}