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!)