split "certbot.tests" into python3-certbot-tests subpackage on EPEL 8
    
    RHEL ships "python3-mock" only as part of the "PowerTools" repo which is not
    enabled by default.
    
    The dependency on "python3-mock" is only present because of the Python
    package "certbot.tests" which provides testing helpers. That code is
    imported by other certbot plugins to run their test suite.
    
    Moving the dependency into a subpackage means that most users won't
    have to enable the PowerTools repo.