fix listener.py and subscriber.py

[joshuamorton]

They currently rely on a directory, we should fix that.

[irapha]

More information, from the original issue: gtagency/buzzmobile#185

Currently mock_node and check topic will fail if tests are run from the
wrong directory because the paths to listener and publisher are hardcoded.
Lets fix that shall we.

[joshuamorton]

Manually tested on pyrostest-test branch of buzzmobile, until we can get some better tests here.