📝 add test for previously introduced bugfix

Author: timfennis

tests/programs/900_bugs/bug0004_shadowing_bug.ndct

@@ -0,0 +1,9 @@
+--PROGRAM--
+fn foo(bar) {
+  let bar = bar + 1; // this used to produce an error in an older version
+}
+
+foo(5);
+print("ok");
+--EXPECT--
+ok