lib: remove redundant code, add tests in timers.js

insert() is only called from one place where there is already a check
that msecs is greater than or equal to zero, so do not repeat the check
inside insert().

timers.active() is not documented and should not be exposed, but since
it is exposed for now, let's test it.

PR-URL: https://github.com/nodejs/node/pull/3143


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>