actually set a dependency between kcrashtest and test_crasher

Summary:
the former uses the latter to actually conduct its test. without the
declared dependency make kcrashtest would not produce a working
result because it'd be missing the test_crasher helper

Test Plan: make kcrashtest now builds both targets

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25154