File Dialog: fix testSelectUrl() again, i.e. selectUrl() should set the directory too

Summary:
This was fixed initially in commit 7bbbd93c (https://phabricator.kde.org/D3796)
when the unittest was added, and broken later in commit bfd41a95
(https://phabricator.kde.org/D4193, bug 374913) which says Qt takes care
of taking the initial directory when calling selectUrl(). I cannot see
that in the Qt code, where selectUrl only calls selectFile_sys.

This breaks bug 374913 again (initial directory for remote files), but the
right fix for that is https://codereview.qt-project.org/235473

Test Plan: unittest

Reviewers: arichardson, anthonyfieroni, elvisangelaccio, plasma-devel, broulik

Differential Revision: https://phabricator.kde.org/D14440