add test for async move in a doctest

Author: QuietMisdreavus

src/test/rustdoc/async-move-doctest.rs

@@ -0,0 +1,14 @@
+// compile-flags:--test
+// edition:2018
+
+// prior to setting the default edition for the doctest pre-parser, this doctest would fail due to
+// a fatal parsing error
+// see https://github.com/rust-lang/rust/issues/59313
+
+//! ```
+//! #![feature(async_await)]
+//!
+//! fn foo() {
+//!     drop(async move {});
+//! }
+//! ```