add test for stripped nested outline module

src/test/ui/parser/stripped-nested-outline-mod-pass.rs

@@ -0,0 +1,13 @@
+// Expansion drives parsing, so conditional compilation will strip
+// out outline modules and we will never attempt parsing them.
+
+// check-pass
+
+fn main() {}
+
+#[cfg(FALSE)]
+mod foo {
+    mod bar {
+        mod baz; // This was an error before.
+    }
+}