http://doc.crates.io/manifest.html#integration-tests
Cargo will not automatically compile files inside subdirectories of tests, but an integration test can import modules from these directories as usual. For example, if you want several integration tests to share some code, you can put the shared code in tests/common/mod.rs and then put mod common; in each of the test files.
I agree it should be easier to find. Took me fifteen minutes to find that, and I'm not quite new. (One or the other of us could make up a documentation PR; those tend to be well received.)