autotests: Make testXdgShellClientRules more robust to CI being heavily loaded

FAIL!  : TestXdgShellClientRules::testSizeApply() 'surfaceConfigureRequestedSpy->wait(10)' returned FALSE. ()
   Loc: [/home/jenkins/workspace/Plasma/kwin/kf5-qt5 SUSEQt5.15/autotests/integration/xdgshellclient_rules_test.cpp(731)]

If CI is moderately loaded, 10ms may not be enough. I don't remember why
10ms timeout was added.