cmd/golangorg: add test cases for recent release history

A following CL will make changes to how release content summary is
represented to support the new security policy, and migrate past
entries. Before doing that, add test cases for the recent releases,
so the upcoming diff will be more informative during review.

For golang/go#51719.
Updates golang/go#38488.

Change-Id: I7d26ac767e69c1ef73a94586f7c166878599864d
Reviewed-on: https://go-review.googlesource.com/c/website/+/393356
Trust: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Trust: Carlos Amedee <[email protected]>

Author: dmitshur