test: make crypto.timingSafeEqual test less flaky

The `crypto.timingSafeEqual` test still seems to be a bit flaky. This
makes a few changes to the test:

* Separates the basic usage and the benchmarking into different tests

* Moves the timing-sensitive benchmark function into a separate module,
and reparses the module on every iteration of the loop to avoid shared
state between timing measurements.

PR-URL: https://github.com/nodejs/node/pull/8456


Reviewed-By: James M Snell <jasnell@gmail.com>