appstreamtest: handle non-installed programs

Summary:
If you just built the software without installing it and then run ctest,
this will systematically fail while trying to read install_manifest.txt.
With this patch this case is now handled gracefully, not forcing to
install to have a test suite which fully passes.

Reviewers: Frameworks, apol

Differential Revision: https://phabricator.kde.org/D3860