Add regression test for unstable items

Author: GuillaumeGomez

tests/rustdoc-js/unstable.js

@@ -0,0 +1,13 @@
+// exact-check
+
+// This test ensures that unstable items are sorted last.
+
+const EXPECTED = [
+    {
+        'query': 'bar',
+        'others': [
+            { 'path': 'unstable', 'name': 'bar2' },
+            { 'path': 'unstable', 'name': 'bar1' },
+        ],
+    },
+];

tests/rustdoc-js/unstable.rs

@@ -0,0 +1,8 @@
+#![feature(staged_api)]
+#![stable(feature = "another", since = "1.0.0")]
+
+#[unstable(feature = "tadam", issue = "none")]
+pub fn bar1() {}
+
+#[stable(feature = "another", since = "1.0.0")]
+pub fn bar2() {}