Fix running the test after requiring ECM 5.46.0

Summary:
300d60bc 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

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D17511