Pi is a post-fix, stack-based language. That means that to call a function (or a built-in operation), you push the arguments then call the operation
1 2 + => 3
The general idea here is to exercise all the functionality of the Pi language via a set of scripts.
There is also a Test called PiTestRaw which performs many tests via GoogleTest, which enable for simpler debugging. Hopefully by the time you read this, that will not be necessary because all the script based tests in this folder will be passing.