Fix temporary path in importer test unit

Summary: Add missing slash.  Without this slash an additional directory is created out of the scope of the QTemporaryDir.  This directory is hence not deleted at the end of the test.

Test Plan: Run importertest and verify that no leftover temporary directories are left in /tmp.

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: tommo, ngraham, #gwenview

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D24874