test: relax timing constraints for child process

With additional load in the system, the child process which runs sleep
command takes more time to run - typically slightly above 1 second,
but above 2 seconds under stress.

While the intent of the test is to test the functionality of spawnSync
and the child process in general, in effect it is testing the system
command sleep, and further, it's responsiveness.

Since from the name the purpose of the test seems to be unrelated to
the sleep behaviour, I believe a more meaningful assertion would be to
see the time taken is more than 1 second.

Reviewed-By: Michael Dawson <mhdawsonibm@gmail.com>
PR-URL: https://github.com/joyent/node/pull/25291