Unittest closure actions

This pull request implements xp-framework/rfc#283 and adds a new class `unittest.actions.VerifyThat`.

```php
class MyTest extends \unittest\TestCase {
  #[@test, @action(new \unittest\actions\VerifyThat(function() {
  #   return ...
  #}))]
  function verify_something_works() {
    $this->assertEquals(...);
  }
}
```