add test case for shadowing of System by class member

semantic/system_shadowing.java

+
+class SystemShadowing {
+    public SystemShadowing System;
+    public SystemShadowing  out;
+
+    public void println(int x) { }
+
+    public void nonstatic() {
+        System.out.println(23);
+    }
+
+    public static void main(String[] args) {
+        SystemShadowing x = new SystemShadowing();
+        x.nonstatic(); /* expected to throw NullPointerException at runtime due to shadowing */
+    }
+}