Fix running the test after requiring ECM 5.46.0
300d60bc7961d6ecff5d58e8fc0f697befac62fb raised the ECM requirement
to 5.46.0 but requiring at least 5.38 comes with a different build
dir layout, helping to test uninstalled apps. This caused the test
to be skipped because knumbertest was not found.
Test Plan: make && make test in a clean build dir
Reviewers: mlaurent, kossebau
Differential Revision: https://phabricator.kde.org/D17511