The TestRunner in question is for FlexUnit, and this seems fairly simple, yet almost impossible for me to do.
To eliminate the possibility of it being the build script I'm using, I downloaded the sample FlexUnit CI project, which contains a sample build script for use. Unfortunately, however, the project complains that no such SWF file exists. I know for a fact that this is because the mxmlc command doesn't actually compile it even though it says compilation was successful.
If anybody has any ideas (or needs anything posting), please let me know, it's driving me insane!