[kdirlistertest] Wait a little longer for the lister to finish

Summary:
Wait a little longer and let m_dirLister finish before checking it has finished.

Test Plan:
Before:
FAIL! : KDirListerTest::testConcurrentListingAndStop() Compared values are not the same

Actual   (m_dirLister.spyCompleted.count()): 0
Expected (1)                               : 1
Loc: [/home/jenkins/workspace/Frameworks/kio/kf5-qt5 SUSEQt5.10/autotests/kdirlistertest.cpp(798)]

After (at home):
Totals: 32 passed, 0 failed, 0 skipped, 0 blacklisted

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16349