in automatic tests, find python interpreter and use it for python test scripts

Summary:
in automatic tests, find python interpreter and use it for python test scripts
this way we do not use /usr/bin/env that may or may not work

Test Plan: Tested only on linux. I have no other platforms at the moment. Not sure for windows.

Reviewers: aacid, awilcox, Frameworks, dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6979