[programming][rust] How to write and use tests

Use assert* macros to test code.
- Built-in: https://doc.rust-lang.org/std/#macros
- External (from the claim library): https://svartalf.info/posts/2020-03-13-assertion-macros-for-rust/
Unit tests go into the same file as the actual code in src/*.rs
The annotation of #[cfg(test)] tells $ cargo build to not build the tests. Instead, use $ cargo test to build and run tests automatically.

#[cfg(test)]
mod tests {
    #[test]
    fn first_unit_test() {
        // Add unit test code here.
    }
}

Use the annotation [#test] to denote that the function is a test. Some functions may be used for setup which are not unit or integration tests.
Integration tests go into separate files at tests/integration_tests.rs.
Use $ cargo test -- integration_tests to only run a specific test in the tests/ directory.
Use $ cargo test <FUNCTION_NAME> to only run a specific test function.
For time-consuming tests, add the [#ignore] annotation after the [#test] annotation to disable the test by default. Run it by using $ cargo test -- --ignored.

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