Let tests use the baloo_filemetadata_temp_extractor from the build dir

Summary:
Without adding the build directory to the PATH,
QStandardPaths::findExecutable will return the temp extractor from the
system, i.e. testing the wrong executable.

Test Plan: ctest

Reviewers: Frameworks, Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D19268