test: reduce sequential/test-fs-watch flakiness

The fs.watch test's write events sometimes aren't produced on OS X,
possibly because of a fsevents race condition. This patch gives delays
the writing a total of 20ms, which makes the test pass consistently.

PR-URL: https://github.com/iojs/io.js/pull/1275


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>