[FileWatchTest] Extend coverage to config updates
Summary:
Currently, config changes are not covered in the
tests, add a test case
Test Plan: ctest -R filewatchtest
Reviewers: Baloo, ngraham, meven
Reviewed By: meven
Subscribers: meven, kde-frameworks-devel
Tags: Frameworks, Baloo
Differential Revision: https://phabricator.kde.org/D29190