Parallel tests, and the fixture that could not be

Eight parallel processes, and one test group that writes to a fixed path on disk.

#[Group('serial')]
final class ExportFileTest extends TestCase
{
    // writes to storage/app/exports/monthly.csv, which
    // is a fixed path in the application's own config
}

// and the pipeline:
//   vendor/bin/paratest --exclude-group=serial
//   vendor/bin/phpunit --group=serial

Two runs — parallel for everything and serial for the exceptions — is more honest than making the fixture parallel-safe, because the fixed path is a property of the application rather than of the test. Eleven tests in the serial group take fourteen seconds, which is a rounding error against the ninety.