Issue 49581 - Fix dynamic plugins test suite
    
    Description: Refactor plugins module and fix tests accordingly.
    Divide the tests into three functions: accaptance, memory corrruption
    and stress.  Add absent Task and Plugin objects.
    Add docstrings for every fixed object.
    Move plugin acceptance tests to a separate module in plugins suite
    
    https://pagure.io/389-ds-base/issue/49581
    
    Reviewed by: vashirov, mreynolds, wibrown (Thanks!)