Add test case for unreachable code

Programs that contain unreachable code are semantically valid MiniJava
according to the latest clarification of the language specification.

semantic/unreachable_code.mj

+/*
+ * MiniJava compilers are not allowed to reject programs only because
+ * they contain unreachable code, unlike Java compilers.
+ */
+class UnreachableCode {
+    public static void main(String[] args) {
+        int i = 42;
+        return;
+        int j = 23;
+    }
+}