Use XPATH notation to match against flattened nodes

The generated code would look like `<code>impl <a href="...">Foo</a></code>`
which the plain text matcher doesn't match. But by using the XPATH notation, the
nodes are flattened and we can correctly assert that `impl Foo` does not occur
in the generated docs.

src/test/rustdoc/hidden-methods.rs

@@ -28,12 +28,12 @@ pub mod hidden {
 
 // @has foo/struct.Foo.html
 // @!has - 'Methods'
-// @!has - 'impl Foo'
+// @!has - '//code' 'impl Foo'
 // @!has - 'this_should_be_hidden'
 pub use hidden::Foo;
 
 // @has foo/struct.Bar.html
 // @!has - 'Methods'
-// @!has - 'impl Bar'
+// @!has - '//code' 'impl Bar'
 // @!has - 'this_should_be_hidden'
 pub use hidden::Bar;