Developers are free to do their things when this constant contains a certain value. In general, you would use this value for testing purposes. Setting a truthy value to this constant will activates the error reporting mode in PHP.

if (defined('TEST') && TEST) {
    throw new Exception('Yo!');
}

You are not forced to set its value to a literal true to enable the test mode. You can set any value to it and Mecha will take the truthy value as an indicator to activate the test mode. Then, certain extensions can use that value to enable their test mode or to run certain tests that they provide.

if (defined('TEST') && 'x.minify' === TEST) {
    // Run specific tests for `minify` extension!
}