Stabilize test KFileWidgetTest::testDropFile

Summary:
The test introduced in D20838 was not completely stable.
It causes failures on CI :

https://build.kde.org/job/Frameworks/view/Platform%20-%20SUSEQt5.12/job/kio/job/kf5-qt5%20SUSEQt5.12/139/testReport/junit/projectroot/autotests/kiofilewidgets_kfilewidgettest/
https://build.kde.org/job/Frameworks/view/Platform%20-%20SUSEQt5.13/job/kio/job/kf5-qt5%20SUSEQt5.13/7/testReport/projectroot/autotests/kiofilewidgets_kfilewidgettest/

This patch replaces a wait by an appropriate QSignalSpy.

Test Plan: LANG ctest -R kiofilewidgets

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22764