rustdoc: add test cases for hidden enum variants

Author: notriddle

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

@@ -0,0 +1,9 @@
+// @has strip_enum_variant/enum.MyThing.html
+// @has - '//code' 'Shown'
+// @!has - '//code' 'NotShown'
+// @has - '//code' '// some variants omitted'
+pub enum MyThing {
+    Shown,
+    #[doc(hidden)]
+    NotShown,
+}