allows tests to stub out plugins that inherit

This is a mock up of plugins.js for testing.  The code is similar to plugins.js in that it does not just import parent functions.  In fact, in my base object I have a RETRY variable assigned to exports that is used in all inheriting plugins.  So making this change would break that functionality.

On a slightly related note.  There is a call to the register() function in plugins.js that I did not port over.  This broke one of my tests, and it was more mocking than I wanted/needed to do at the time.  In this way, the stub_plugins.js's inherit() function is slightly different.