These tests have an implicit main function added which makes working with extern crate dependancies unintuitive. I suppose rusty_ulid is your own crate, that one is already correctly imported automatically.
It might be that the test generation algorithm on stable and beta has been improved, but hasn't been pushed to stable yet?
I'd link to rustbyeample.com for more information, but the site has been down for a while (for me).
If my google foo doesn't fail me this should be the relevant link: https://rustbyexample.com/testing/doc_testing.html
Also remember that the crates used in tests have to be listed as dev-dependancies if they are not already listed under normal dependancies.
Edit: If the implicit main is actually the problem you can solve it by implementing your own main function, just like you would write into your lib.rs or main.rs files.