test: add checkMethods function for Certificate

This commit adds a checkMethods function so that it can be reused to
avoid duplicating the same code for instance methods, and static
methods of the Certificate object.

PR-URL: https://github.com/nodejs/node/pull/20224


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>