Add test for ignored private doc test

Author: GuillaumeGomez

src/test/rustdoc-ui/coverage/doc-examples.stdout

@@ -1,7 +1,7 @@
 +-------------------------------------+------------+------------+------------+------------+
 | File                                | Documented | Percentage |   Examples | Percentage |
 +-------------------------------------+------------+------------+------------+------------+
-| ...tdoc-ui/coverage/doc-examples.rs |          4 |     100.0% |          2 |      50.0% |
+| ...tdoc-ui/coverage/doc-examples.rs |          4 |     100.0% |          1 |      25.0% |
 +-------------------------------------+------------+------------+------------+------------+
-| Total                               |          4 |     100.0% |          2 |      50.0% |
+| Total                               |          4 |     100.0% |          1 |      25.0% |
 +-------------------------------------+------------+------------+------------+------------+

src/test/rustdoc-ui/private-doc-test.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![deny(private_doc_tests)]
+
+mod foo {
+    /// private doc test
+    ///
+    /// ```ignore (used for testing ignored doc tests)
+    /// assert!(false);
+    /// ```
+    fn bar() {}
+}