Add test to enforce sidebar trait items order

src/test/rustdoc-gui/lib.rs

@@ -47,14 +47,19 @@ pub fn some_more_function<T: fmt::Debug>(t: &T) -> String {
 
 /// Woohoo! A trait!
 pub trait AnotherOne {
+    /// Some func 3.
+    fn func3();
+
     /// Some func 1.
     fn func1();
 
+    fn another();
+    fn why_not();
+
     /// Some func 2.
     fn func2();
 
-    /// Some func 3.
-    fn func3();
+    fn hello();
 }
 
 /// Check for "i" signs in lists!

src/test/rustdoc-gui/trait-sidebar-item-order.goml

+goto: file://|DOC_PATH|/trait.AnotherOne.html
+assert: (".sidebar-links a:nth-of-type(1)", "another")
+assert: (".sidebar-links a:nth-of-type(2)", "func1")
+assert: (".sidebar-links a:nth-of-type(3)", "func2")
+assert: (".sidebar-links a:nth-of-type(4)", "func3")
+assert: (".sidebar-links a:nth-of-type(5)", "hello")
+assert: (".sidebar-links a:nth-of-type(6)", "why_not")