Adds Clojure, Python and Go extensions to `test_identifiers`

The `test_indentifiers` method has fallen out of step with the site and is missing entries for Clojure and Python. As it looks like Go has the most assignments created out of the non-published languages, I've added an entry and associated test for it as well.

I had an initial thought with regard to improving the maintainability of the `submission` test file: as the file formats for test files are common to all languages excepting JavaScript, it might be worth creating the test methods dynamically based on a hash mapping of language to file extensions. I'll have a go at doing this in a subsequent changeset.