Pull request commit testing function: app.lua:testCommit()

*Quote from this function's description:*
> CorsixTH programmers can help the testers of their pull requests by
providing testing code for each of their commits using this function which
testers can execute via the debug console by using the console
command: TheApp:testCommit()

> For each commit in your pull requests, please change the contents of this function to either test the commit's new change set or to print "--- App:testCommit(): No test provided. ---"

I think this function should be added:
1) To making testing pull request commits quicker and easier.
2) So that commit testers can see what the creators of commits haven't tested.