ignore local change event if there is no file

When a chokidar change event occurs on a file that is not located on the Cozy
file system, nothing is done anymore. That looks like a chokidar bug. It looks
like file that finishes with a ~ are wrongly handled.
bump chokidar to 0.12.5
many small improvements and fixes for testing

attempt to make things more tested
-> change test always fail in a super weird way. I extracted it to be able to
run it many times. When I do that, it works well.