Incorporate stfitsdiff into regression tests

[stscijgbot-jp]

Issue JP-3908 was created on JIRA by Melanie Clarke:

In JP-3789, we implemented STFITSDiff to support more concise, informative reports for FITS file differences.  To follow up, we'd like to do some more local testing then get the tool implemented in the existing regression tests.  We'd like to get this in early in the next build cycle so we have plenty of time to catch any potential issues.