test: fix flaky test-timers-blocking-callback

This test was failing on FreeBSD from time to time in the project CI.
The bug the test was written for would guarantee that the timer would
fire at least 100ms late, but the assertion was firing if it was more
than 50ms late.

This changes the assertion to fire when the timer is more than 100ms
late.

I ran a modified version of this test using 0.10.38 (which has the bug)
and 0.10.39 (which has the fix) to confirm that it still fails in the
buggy one and passes in the fixed one.

PR-URL: https://github.com/nodejs/node/pull/9198


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>