Deduplicate doc tests logic

[GuillaumeGomez]

This comes from a discussion in #86230: for the moment, we call libtest from two places: markdown.rs (which is used when we provide a markdown file as input) and doctest.rs (when a rust file is provided as input, the default).

Both calls can be found when grepping with testing::test_main(.

[camelid]

Probably the deduplicated logic should go in doctest.rs.