Add test cases for namespaced crates

[eholk]

This is just to add a few test cases of agreed upon desired behavior for this feature so that future PRs can enable them as they start passing.

I pulled these tests out of #140271.

r? @petrochenkov

cc @b-naber

[petrochenkov]

In the 3-scope model this would import from both my_api(crate) and my_api(open_ns).
Therefore it will introduce names root_function, core and utils into this module.
Need to test this behavior.

Old comment (based on mixing open-ns-my_api.rs and open-ns-mod-my_api.rs):

Therefore it will introduce names utils(mod), root_function and core into this module, but not utils(crate) because it would be shadowed by utils(mod).

It would be good to actually add a test case with scenario like that.

[petrochenkov]

Hm? There's no open-ns-mod-my_api.rs in this test.

[JohnCSimon]

@eholk
ping from triage:
Can you please address the comments from the reviewer?

FYI: when a PR is ready for review, send a message containing
@rustbot ready to switch to S-waiting-on-review so the PR is in the reviewer's backlog.

Thank you.

[b-naber]

@JohnCSimon @eholk is still on vacation, so I address your comment.

This is still waiting on a review (see #142437 (comment)).

[petrochenkov]

I did the re-review and marked this as waiting on author because some comments (#142437 (comment), #142437 (comment)) are still unaddressed.
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