Add header keyword value quality checks to unit tests

[jaymedina]

Add header keyword value quality checks to unit tests for make_cube.py.

There was an incident where the EXT1 header keyword NAXIS2 did not reflect the correct number of FFIs that were added to the cube. We are still unsure of what happened, but as a preventative measure, it would be good to have some checks in place to verify the quality of the primary and image header keyword values.