How to write tests for the Scala source code isn't terribly well documented yet, but it is possible to write tests for the 'partest' test application by
putting a Scala test file in the 'test/files/jvm' directory. This is a Scala 'object' file with a 'main' method which prints to the standard output;
putting a '.check' text file in the same directory. This has the same file name stem as the Scala file, but has '.check' as its extension. The text in this file is compared with the output of the Scala file by 'partest'.
At least, that's how I'm told it works. I'm trying to get one working now.