gvfsd-trash sets an inotify watch on mntDir and stat()s every file that is
created, racing with the test logic.

Create our files in a subdir to avoid that, as gvfsd-trash does not monitor
subdirs.

Fixes: https://github.com/hanwen/go-fuse/issues/478
Change-Id: I8e297a16af7fdd8b8c9574d456bbf43d36eddd67