Migrate build system to use find_package in autotests/ki18n_install

Summary:
The current build system used an incomplete integration of the KI18n
macro file.

Test Plan: compiled and did run make test to check autotests

Reviewers: apol, ilic, dfaure

Reviewed By: dfaure

Subscribers: dfaure, bcooksley, ltoscano, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13743