test for kevent buffer data race from @tilaks
test for kevent buffer data race from @tilaks

`go test -test.run=TestConcurrentRemovalOfWatch -test.cpu=1,1,1,1,1 -race`

from CL https://codereview.appspot.com/103300045/

fsnotify/fsnotify,fsnotify/fsnotify,treejames/fsnotify,qida/fsnotify,mingshun/fsnotify,fsnotify/fsnotify,beni55/fsnotify,scalingdata/fsnotify-v1,omeid/fsnotify,jsix/fsnotify