Add rustdoc index page test for future deprecation attributes

Author: varkor

src/test/rustdoc/deprecated-future.rs

@@ -1,5 +1,7 @@
 #![feature(deprecated)]
 
+// @has deprecated_future/index.html '//*[@class="stab deprecated"]' \
+//      'Deprecated'
 // @has deprecated_future/struct.S.html '//*[@class="stab deprecated"]' \
 //      'Deprecated since 99.99.99: effectively never'
 #[deprecated(since = "99.99.99", note = "effectively never")]