autotests: Find perl executable on all platforms

Summary: Attempt to fix unit tests on FreeBSD.

Test Plan: Unit tests are able to run Perl script on Linux. (Would be better to test this on all platforms, but I can't do that easily.)

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure, apol, Frameworks, kde-pim, bcooksley

Tags: KDE PIM, Frameworks

Differential Revision: https://phabricator.kde.org/D25638