[autotests] Register KWin::Deleted* in SlidingPopupsTest

Summary:
QSignalSpy is not happy about KWin::Deleted*:

QWARN  : SlidingPopupsTest::testWithOtherEffectWayland(wobblywindows, slide)
QSignalSpy: Unable to handle parameter 'deleted' of type 'KWin::Deleted*' of
method 'windowClosed', use qRegisterMetaType to register it.

Test Plan: Ran the test, haven't noticed any warnings.

Reviewers: KWin, broulik

Reviewed By: broulik

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15730