[CentOS] Installing yesterday's CentOS (or how to install the patch/package set from 3 weeks ago)

Hello,

Let's say your operating policy is "no patch updates without testing
first in the test environment".   Let's say it takes you 3 weeks to
test.  Over the course of the 3 weeks, the repo changes (new
packages added, old removed).

Is there a way to "freeze" a set of packages so that when I
run "yum update" on a Prod server it'll get the same package
and patch set as the Test server did 3 weeks ago?

It's been suggested to maintain a local mirror, and take rsync
snapshots of it daily, so then you can point the end node to a
particular repository.

What other solutions are there?

Best,
-at