Add test for where clause order

src/test/rustdoc/where-clause-order.rs

+#![crate_name = "foo"]
+
+pub trait SomeTrait<Rhs = Self>
+where Rhs: ?Sized
+{}
+
+// @has 'foo/trait.SomeTrait.html'
+// @has - "//div[@id='impl-SomeTrait%3C(A%2C%20B%2C%20C%2C%20D%2C%20E)%3E-for-(A%2C%20B%2C%20C%2C%20D%2C%20E)']/h3" "impl<A, B, C, D, E> SomeTrait<(A, B, C, D, E)> for (A, B, C, D, E) where A: PartialOrd<A> + PartialEq<A>, B: PartialOrd<B> + PartialEq<B>, C: PartialOrd<C> + PartialEq<C>, D: PartialOrd<D> + PartialEq<D>, E: PartialOrd<E> + PartialEq<E> + ?Sized, "
+impl<A, B, C, D, E> SomeTrait<(A, B, C, D, E)> for (A, B, C, D, E) where
+    A: PartialOrd<A> + PartialEq<A>,
+    B: PartialOrd<B> + PartialEq<B>,
+    C: PartialOrd<C> + PartialEq<C>,
+    D: PartialOrd<D> + PartialEq<D>,
+    E: PartialOrd<E> + PartialEq<E> + ?Sized
+{}