Add a --show-diff argument to unit tests

[ehmatthes]

One of the most frequent test failures is the check_reference_file() assertion after calling filecmp.cmp(). To troubleshoot this, I need to open the test artifact and look at the file that was generated, and compare it to the reference file. This comparison could be automated.

• Add a flag that automatically shows a diff of the files that failed the comparison.

Notes

• May require use of -x, and -s.