Add unittests

[Levi-Leah]

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.