[3.11] [3.12] gh-108303: Move all certificates to `Lib/test/certdata/` (GH-109489) (GH-109682) (#110646)

[3.12] gh-108303: Move all certificates to `Lib/test/certdata/` (GH-109489) (GH-109682)

* gh-108303: Move all certificates to `Lib/test/certdata/` (GH-109489)
(cherry picked from commit e57ecf6bbc59f999d27b125ea51b042c24a07bd9)

Python 3.12 backport: update also `test_nntplib`.

(cherry picked from commit c2d542b42cd109d81c0308f9c4437c38ac74d2e0)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: T. Wouters <thomas@python.org>