Add `api` prefix as a top level route in the spec.

Now that it has been removed from the rails routes. But it still needs
to be a reserved top-level word, so the tests should know about this.

spec/lib/gitlab/path_regex_spec.rb

@@ -86,7 +86,7 @@ describe Gitlab::PathRegex, lib: true do
       route.split('/')[1]
     end.compact.uniq
-    words + ee_top_level_words + files_in_public
+    words + ee_top_level_words + files_in_public + Array(API::API.prefix.to_s)
   end
   let(:ee_top_level_words) do