Add integration test that checks correctness with xlsx validator

[jappeace]

My comment at verbatim:

To validate the result is correct xml:                                                                     

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docker run -v $(pwd):/app/xlsx-validator/xlsx -it vindvaki/xlsx-validator xlsx-validator xlsx/yourfile.xlsx
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TODO: package that program for nix and make a unit test out of it.                                         
it's *really* usefull.                                                                                     

This will put the xlsx project root in the current working                                                 
directory of xlsx validator,                                                                               
allowing you to run that program on the output from                                                        
streaming (which in this example was written to out/out.xlsx).                                             
This gives a much more descriptive error reporting than excel.            

It's this program: https://github.com/vindvaki/xlsx-validator