How to move tests into a separate file for binaries in Rust's Cargo?

Having created a new binary using Cargo

cargo new my_binary --bin

Now adding a function to my_binary/src/main.rs can be used for a test, eg:

#[test]
fn my_test() {
    println!("Test OK");
}

Now cargo test -- --nocapture runs the test as expected.

My question is, whats the most straightforward way to move these tests into a separate file?

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I tried to define a crate and move the source to tests/main.rs but it wasn't automatically found.

Probably am missing something, but didn't manage to get this working following online docs.