A process is not terminated even if a script is completed

[makenowjust]

What is the problem?

The following script is to check a simple regexp. It prints a safe result, but it is not terminated after the printing.

const { check } = require('recheck');

(async () => {
  console.log((await check('a', '')).status);
})();

How to fix the problem?

The reason of this bug is that calling subprocess.unref is missing.
We should count running checks and it calls unref correctly when all checks are done.