autotests: don't fail appstream test because of anything on stderr

Summary:
ERROR_VARIABLE simply means where stderr goes, this includes any qDebug,
which surely doesn't mean failure.

Test Plan: QT_LOGGING_RULES='*=true' ctest. See also CI constantly failing...

Reviewers: sitter, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20749