Add unittests

Add unittests for validation scripts:

- pcvalidator.py
- pcchecks.py
- pcprvalidator.py
- pcyamlchecks.py
- pcutil.py

Shared scripts:

- pcutil.py

And preview scripts:
- pcbuild.py

Implementation of unittests might trigger a rewrite of some of the scripts (e.g. pcutil.py. we open the yml file 4 times there, we can get away with just returning it, perhaps) and reorg.
