Fix filetypestest

Summary:
Concatenating strings results in paths like: ".qttestconfig" instead of
the expected ".qttest/config". QMimeDatabase doesn't check the cache
files on every call, but only if at least 5 seconds have passed.

The failed tests can be seen here:
https://build.kde.org/job/Plasma%20kde-cli-tools%20kf5-qt5%20SUSEQt5.10/lastCompletedBuild/testReport/(root)/TestSuite/filetypestest/
and:
https://ci.debian.net/data/autopkgtest/unstable/amd64/k/kde-cli-tools/266696/log.gz

Reviewers: Plasma, kossebau, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12781