Add PHPUnit Polyfills library

It's been a while since I've used Tester. As I've been setting it up I've noticed this error:

```
Error: The PHPUnit Polyfills library is a requirement for running the WP test suite.
```

We could automate this inside tester by adding the Chassis Composer extension and using it to automagically install https://github.com/Yoast/PHPUnit-Polyfills and require the `vendor/autoload.php`