Add regression test for rust-lang#136899

Author: GuillaumeGomez

tests/rustdoc-ui/hashtag-doctest.rs

@@ -0,0 +1,17 @@
+// This test ensures that `##` are not emitting a warning when generating
+// docs with the 2024 edition (or any edition).
+// Regression test for <https://github.com/rust-lang/rust/issues/136899>.
+
+//@ check-pass
+//@revisions: edition2021 edition2024
+//@[edition2021] edition:2021
+//@[edition2024] edition:2024
+
+#![deny(warnings)]
+
+//! Test
+//!
+//! ```
+//! ##[allow(dead_code)]
+//! println!("hello world");
+//! ```