testing: workaround pytest parser but in pytest-atf-wrapper.

[melifaro]

Summary

Problem:

parsing of plugin args breaks after having any path-like argument (--atf --atf-file /dev/null works, but --atf --atf-file /dev/null --atf-cleanup does not).

Pytest issues: 3097, 6904 and probably others.

This diff implements the suggested workaround - passing values in form --arg=value instead of --arg value.