Add a rustdoc test for future rustc_deprecated attributes

Author: varkor

src/test/rustdoc/rustc_deprecated-future.rs

@@ -0,0 +1,11 @@
+#![feature(staged_api)]
+
+#![stable(feature = "rustc_deprecated-future-test", since = "1.0.0")]
+
+// @has rustc_deprecated_future/index.html '//*[@class="stab deprecated"]' \
+//      'Deprecation planned'
+// @has rustc_deprecated_future/struct.S.html '//*[@class="stab deprecated"]' \
+//      'Deprecating in 99.99.99: effectively never'
+#[rustc_deprecated(since = "99.99.99", reason = "effectively never")]
+#[stable(feature = "rustc_deprecated-future-test", since = "1.0.0")]
+pub struct S;