test: Timeout#unref() does not return instance

Timeout#unref() call returns undefined, not this. The test already
worked before, because the interval was still unref'd, and the test also
succeeds without clearing the interval.

PR-URL: https://github.com/joyent/node/pull/9171


Reviewed-by: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Timothy J Fontaine <tjfontaine@gmail.com>