Improve rustdoc::private_doc_tests tests

- Ensure standard public items are accepted
- Ensure public items not re-exported from private modules are denied

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

@@ -0,0 +1,11 @@
+#![deny(rustdoc::private_doc_tests)]
+
+mod foo {
+    /// private doc test
+    ///
+    /// ```
+    /// assert!(false);
+    /// ```
+    //~^^^^^ ERROR documentation test in private item
+    pub fn bar() {}
+}

src/test/rustdoc-ui/private-public-item-doc-test.stderr

@@ -0,0 +1,18 @@
+error: documentation test in private item
+  --> $DIR/private-public-item-doc-test.rs:4:5
+   |
+LL | /     /// private doc test
+LL | |     ///
+LL | |     /// ```
+LL | |     /// assert!(false);
+LL | |     /// ```
+   | |___________^
+   |
+note: the lint level is defined here
+  --> $DIR/private-public-item-doc-test.rs:1:9
+   |
+LL | #![deny(rustdoc::private_doc_tests)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/rustdoc-ui/public-reexported-item-doc-test.rs

@@ -2,7 +2,9 @@
 
 #![deny(rustdoc::private_doc_tests)]
 
-mod foo {
+pub fn foo() {}
+
+mod private {
     /// re-exported doc test
     ///
     /// ```
@@ -11,4 +13,4 @@ mod foo {
     pub fn bar() {}
 }
 
-pub use foo::bar;
+pub use private::bar;