rustdoc: Add a test for should_fail in doctests

Closes #23744

Author: alexcrichton

src/test/rustdoc/issue-23744.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags:--test
+
+/// Example of rustdoc incorrectly parsing <code>```rust,should_panic</code>.
+///
+/// ```should_panic
+/// fn main() { panic!("fee"); }
+/// ```
+///
+/// ```rust,should_panic
+/// fn main() { panic!("fum"); }
+/// ```
+pub fn foo() {}