Allow API to run question tests

[EJMFarrow]

• Add a test endpoint that accepts a question, runs its tests and returns the results.
• If a question has PRTs but no tests then a default test using the model inputs will be run looking for a score of 1.
• Allow conditional loading of tests in the question loader.
• Add bulktest front end page to run through all questions in a folder and sub-folder.
• Fix an issue in the question loader related to empty elements being interpreted as empty strings rather than nulls.
• Use stack_string in the front end to allow translation.
• Fix security issue with Slim dependency by upgrading.
• Fix: Replace absolute URLs for API calls with relative URLs.
• There are changes to questiontest.php (separate some code into a new function so it can also be used by the API) and questiontestresult.php (a new function only used by the API) in the main code.
• Add extra tests and update documentation.