You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: test_util.rs
    path of file: ./repos/tectonic/src
    
    the code of the file until where you have to start completion: // Copyright 2016-2020 the Tectonic Project
// Licensed under the MIT License.

//! This inelegant module helps with testing.
//!
//! We would like it if all of our tests -- unit tests
    
pub fn test_path(parts: &[&str]) -> PathBuf {
    let mut path = PathBuf::from(env::var_os(TEST_ROOT_ENV_VAR).expect(
        "Tectonic testing infrastructure cannot be used without \
         setting the magic test-root environment variable",
    ));
    path.push(parts.iter().collect::<PathBuf>());
    path
}