Add regression test for #136899

Author: GuillaumeGomez

tests/run-make/rustdoc-2024-hash-doctest/foo.rs

@@ -0,0 +1,6 @@
+//! Test
+//!
+//! ```
+//! ##[allow(dead_code)]
+//! println!("hello world");
+//! ```

tests/run-make/rustdoc-2024-hash-doctest/output.stdout

@@ -0,0 +1,13 @@
+// This test ensures that `##` are not emitting a warning when generating
+// docs with the 2024 edition.
+// Regression test for <https://github.com/rust-lang/rust/issues/136899>.
+
+use run_make_support::{bare_rustdoc, diff};
+
+fn main() {
+    let out = bare_rustdoc().input("foo.rs").arg("-Zunstable-options").edition("2024").run().stdout_utf8();
+    diff()
+        .expected_file("output.stdout")
+        .actual_text("actual", out)
+        .run();
+}