To the best of my knowledge this is not possible due to the ant build, but there might be some way that I am just not familiar with.
If you are launching the tests in Flash Builder, they can be launched in debug mode and you can accomplish the behavior you mention. It is the ant component here that becomes the wild card.
It is possible!
Flash Builder compiles swf in bin-debug folder for debugging. If you copy and override this file with the one you built using ant in your build process, Flash Builder will use that file instead.
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