only benchmark once

Summary:
benchmarking N times slows down the test considerably for what feels like
zero testing advantage. getting just some rough benchmark data should
be entirely sufficient for a unit test. when more detailed
benchmarking is required one can easily switch the benchmark macro

Test Plan: 6 times faster test pass

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22402