math/rand/v2: clean up regression test

math/rand/v2: clean up regression test

Add more test cases.
Replace -printgolden with -update,
which rewrites the files for us.

For #61716.

Change-Id: I7c4c900ee896042429135a21971a56ebe16b6a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/516858
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>