Adding unit testing to paperless-ai

[fernanfs]

During preparations on contributing to paperless-ai, I've discovered, that there is a lack of unit testing.
Unit testing makes contributions easier and avoids regressions. In addition, it makes the intention of methods/functions clear by providing test cases, detailling the input parameters and output expectations.

This pull request:

• adds jest as a dependency for unit testing
• renames the existing run target test to dev. Starting the development instance can now be done by npm run dev, which is common to most node based applications
• adds a run target test which executes the unit tests
• Adds a number of testcases as a first starting point

[clusterzx]

This pull request has been marked as stale due to inactivity. Please update it to keep it open.

[fernanfs]

Hey @clusterzx !
Is there anything I should add to make this worth merging? I'd like to contribute more content, but would like to have the chance of incorporating unit tests for those changes.