TEST: Improve Testing Implementation (for configuration option testing)

[mmcky]

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It would be nice not to duplicate all the source files from base to no_inline_exercise test set. I will put an issue together to improve on this. What would be better is to use -D overrides on a common set of files with additional reference ipynb files.

configuration = {
  base : [("exercise_inline_exercises = False", ipynb/<reference ipynbs)]
}

We could then have a matrix that runs:

sphinx-build  BASE BASE/_build -D exercise_inline_exercises = False

and run that over base test set with different configuration options for comparison with different reference ipynb files.