Strenghten strip-enum-variant.rs test

Author: GuillaumeGomez

src/test/rustdoc/strip-enum-variant.no-not-shown.html

@@ -0,0 +1 @@
+<ul><li><a href="#variant.Shown">Shown</a></li></ul>

src/test/rustdoc/strip-enum-variant.rs

@@ -2,6 +2,8 @@
 // @has - '//code' 'Shown'
 // @!has - '//code' 'NotShown'
 // @has - '//code' '// some variants omitted'
+// Also check that `NotShown` isn't displayed in the sidebar.
+// @snapshot no-not-shown - '//*[@class="sidebar-elems"]/section/*[@class="block"][1]/ul'
 pub enum MyThing {
     Shown,
     #[doc(hidden)]