test: fix time resolution constraint

The modification time of a file is assumed to happen at the
exact time when it was requested. As the utime API specification
delcares that the resolution of the result is 1 second,
relax the constrain to 1 second helps the test case to be
robust and consistent under different load conditions in the system

PR-URL: https://github.com/nodejs/node/pull/3981


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>