test: fix flaky unrefed timers test

Remove unnecessary `setImmediate()` that causes a minor race condition.
Stop the test after 3 occurrences rather than 5 to allow for slower
hosts running the test in parallel with other tests.

Fixes: https://github.com/nodejs/node/issues/4559
PR-URL: https://github.com/nodejs/node/pull/4599


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>