If you want to find a file next to your test and run it as a script, you can just do this:
from twisted.python.modules import getModule
script = getModule(__name__).filePath.path
# ...
reactor.spawnProcess(..., script, ...)
You can also use this to support storing your code in a zip file, although invoking it with Python becomes a little more difficult that way. Have you considered just using python -m
?