Regression tests for PBAssign.py

Add a test_regression.py script.

This script runs the various PBxplore programs with various argument, and makes
sure the output is the expected one. The aim is to check that the program is
not broken during development.

Be careful this script does not test that the output is right. It just test that
the output is the expected one based on previous a previous version.

To run this test suite, you can either run this script without arguments or use
the nose package (https://nose.readthedocs.org). The latter option gives a more readable output, as stdout
is captured, and is displayed only if a test fails:

    $ nosetests test_regression.py
    ........
    ----------------------------------------------------------------------
    Ran 8 tests in 26.429s
    
    OK