Rustpkg only builds one test.rs file per crate

[sfackler]

If I have a project that looks like

foo/
    bar/
        mod.rs
        test.rs
    lib.rs
    test.rs

and run

rustpkg test foo

the only tests run are the ones in bar/test.rs.

A concrete example is in https://github.com/sfackler/rust-postgres, where only the tests in pool/test.rs are run.

[catamorphism]

Related to #7240

[catamorphism]

We plan to remove the "alternate crate root" way of testing rustpkg packages (see #9003), so this is going to be a non-issue. The more general issue is #7240, which is still open. Closing.