[Mac] fast/events/mouse-cursor-image-set.html is flaky
https://bugs.webkit.org/show_bug.cgi?id=116244

Add a flaky test expectation on Mac.

* platform/mac/TestExpectations:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150208 268f45cc-cd09-0410-ab3c-d52691b4dbfc