[DOC] Add unit test instructions
Per @aaron.bray 's comment, unit test documentation should be added under How to perform verification and validation
Related to https://github.com/openjournals/joss-reviews/issues/4929
Per @aaron.bray 's comment, unit test documentation should be added under How to perform verification and validation
Related to https://github.com/openjournals/joss-reviews/issues/4929