Add another test case for shadowing behavior of the System identifier

semantic/system_shadowing.invalid.mj

+/*
+ * This should be rejected by a MiniJava compiler like it is by the
+ * Java compiler as the class "System" shadows the default System
+ * declaration, but cannot have a static field named "out".
+ */
+class System {
+    public static void main(String[] args) {
+        System.out.println(42);
+    }
+}