Don't leave lingering files in doc tests

Closes #11234

src/libstd/io/mod.rs

@@ -53,6 +53,8 @@ Some examples of obvious things you might want to do
     # let _g = ::std::io::ignore_io_error();
     let mut file = File::create(&Path::new("message.txt"));
     file.write(bytes!("hello, file!\n"));
+    # drop(file);
+    # ::std::io::fs::unlink(&Path::new("message.txt"));
     ```
 
 * Iterate over the lines of a file

src/test/compile-fail/lint-missing-doc.rs

@@ -50,6 +50,7 @@ pub trait A {
     /// dox
     fn foo_with_impl() {}
 }
+#[allow(missing_doc)]
 trait B {
     fn foo();
     fn foo_with_impl() {}