fix autotest

Summary:
now the look&feel kcm actually checks for the colorscheme
file existence. therefore the test has to make sure
that such a file actually exists

Test Plan: autotest works now

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4297