#1686 Python3: Unit Tests for /common (p3/tests)

[Al von Ruff]

Includes:

• New directory for unit tests.
• Set of tests targeting /common
• Makefile changes to support new directory
• Fix to isbn.py for remainder calculation
• Fix to library.py for cgi escape usage under Python3
• Fix to ISFDBdaysFromToday as it was always 1 day off