[3.14] gh-135376: Fix and improve test_random (GH-135377) (GH-135680)

* Remove duplicated code. Tests for Random and SystemRandom now share
  the code.
* Move implementation agnostic tests that was only run for SystemRandom,
  so they are now run for Random too.
* Add tests for __index__() support.
* Add tests for randint().
(cherry picked from commit c55512311b7cb8b7c27c19f56cd8f872be29aedc)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>