While the accepted answer works for the OP's purposes, and while the second answer is correct for updating the python sys.path variable, I think, if the OP weren't able to use the accepted answer (because, say, there was a policy against modifying the OS PATH variable on build/test machines), something like this SO answer would be what they are looking for. Summarizing the simple case here, to change the OS PATH environment variable:
app_path = os.path.join(root_path, 'other', 'dir', 'to', 'app')
os.environ["PATH"] += os.pathsep + app_path
At least, this is what I was hoping to find when I read the question.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…