test: make spawnSync() test robust

The test had checked that a timer fired within a period after
spawnSync() returns. The result was a test that sometimes was
flaky.

Because there's no guarantee of how long a timer will take
before running, remove the check. There is a check that the
timer runs after spawnSync() so that is sufficient.

PR-URL: https://github.com/nodejs/node/pull/2535
Fixes: https://github.com/nodejs/node/issues/2470


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>