[3.14] gh-141174: Improve `annotationlib._Stringifier` test coverage (GH-141220) (#141358)

gh-141174: Improve `annotationlib._Stringifier` test coverage (GH-141220)

* Test `_Stringifier.__convert_to_ast()` for containers

* Test partial evaluation of `ForwardRef`s in `_Stringifier`
(cherry picked from commit 55ea13231313a2133e6f5a6112409d349081f273)

Co-authored-by: dr-carlos <77367421+dr-carlos@users.noreply.github.com>