test: refactor test-fs-watch

* Exchange 20 millisecond timers for setImmediate().

* Do not attempt to unlink path that will have been guaranteed to be
removed by `common.refreshTmpDir()`

* Do not swallow errors thrown by failed creation of needed test
subdirectory. If that happens, we want to know about it.

* Use `common.isSunOS` in one place where it is applicable

PR-URL: https://github.com/nodejs/node/pull/4776


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>