Add minimal tests

[davanstrien]

It would be good to add at least minimal tests before releasing this publicly

Things to test

• e2e XML produces expected metadata fields
• cli
• ???

There may be some opportunities for refactoring to allow for some more unit tests

[andrewphilipsmith]

I just had a discussion with @fedenanni and @thobson88 about this.
Our notes are here https://hackmd.io/KeOzeaMYTOiF37pGUjUl7A

We suggested these initial tests:

• Does the output file contain the correct text for a known input file?
• Does alto2txt report correctly report the number of input files that have been processed?
• Does alto2txt count the number of articles in a METS file correctly?
• Does the number of articles found in the (METS file), match the number of output files?
• Does alto2txt throw an error if there are ALTO files missing that are described in a METS file?
• Does alto2txt throw an error if there are orphaned ALTO files