add test to check shadowing behavior of String class

semantic/main_with_local_string_class_compiles.java

+/* String shadows the default String class.
+ * However, Java only detects the lack of a main() at runtime,
+ * failing with 'error: Main method not found'
+ */
+class String { }
+
+class MainWithLocalStringShadowingJavaString {
+    public static void main(String[] args) {}
+}
+