Add more unit tests for routines in common.py

[mkavulich]

This PR is a demo to begin adding more unit test coverage to the CCPP-framework. Since a unit test was recently added for the test_string_to_python_identifier function in common.py, I added unit tests for the remaining functions in that file.

To run these unit tests, simply navigate to the test/unit_tests directory and run the script (with Python 3.7 or higher):

python test_common.py 
.....
----------------------------------------------------------------------
Ran 5 tests in 0.023s

OK

User interface changes?: No

Fixes: No issue specific to this change. This test focuses on the ccpp_prebuild portion of code, but should be a stepping stone to a number of issues relating to capgen unit tests.

Testing:
unit tests: All passed

[gold2718]

Although I have a requested change, it is more of a suggestion.

[climbfuji]

Thanks for adding more tests to the framework! I agree with @gold2718's suggestion. The "That dog won't hunt, Monsignor" test is hilarious :-)