Improve specificity of test

This test is to check that the contents method returns the memoised
@contents when repeatedly called, rather than running line 10
https://github.com/alphagov/government-frontend/blob/6dbefab2ed477a6328f57227494b4561f811f239/app/presenters/content_item/contents_list.rb#L10

The original test checked if the code on line 11 was run, ie the contents_items method.
But the content_items method is called in two places in this module, within both the contents method (which
this test is testing) but also within the show_contents_items? method.This means changes
to the show_contents_items? code unexpectedly breaks this test. This commit reduces the scope
of the test so that unexpected test failures no longer occur.

test/presenters/content_item/contents_list_test.rb

@@ -17,7 +17,7 @@ def body
       end
     end
 
-    @contents_list.expects(:contents_items).returns([{ text: "A heading", id: "custom" }]).once
+    @contents_list.expects(:show_contents_list?).returns(true).once
     @contents_list.contents
     @contents_list.contents
   end