testsuite: Add test for #5582, which passes.

[catamorphism]

Closes #5582

[huonw]

I'm not sure this is correctly working. The follow still fails:

mod foo {
    use vectors = std::vec;
    use vectors::raw;
}

with error: unresolved import. maybe a missing extern mod vectors? error: failed to resolve import vectors::raw``.

I think the test case here works because it's importing vectors into the top of the crate, and use statements are always relative to there.

[catamorphism]

@huonw is right -- this is the wrong test.