xunit_directive: improve docs, tests, some code
    
    Add more information to documentation. Always set xunit file
    as artifact. Use an existing method for creating a success
    rate summary. Simplify tests by reusing data. Don't mock builtin
    open, override it just inside the module under test.
    
    Bump requirements for mock to 2.0.0 to avoid bugs in older version.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D975