Unit Test: Update Test_Upgrader_Skin

```
PHP Strict Standards: Declaration of Test_Upgrader_Skin::feedback() should be compatible with WP_Upgrader_Skin::feedback($string, ...$args) in /tmp/wordpress-master/src/wp-content/plugins/jetpack/tests/php/sync/test_class.jetpack-sync-themes.php on line 41

Strict Standards: Declaration of Test_Upgrader_Skin::feedback() should be compatible with WP_Upgrader_Skin::feedback($string, ...$args) in /tmp/wordpress-master/src/wp-content/plugins/jetpack/tests/php/sync/test_class.jetpack-sync-themes.php on line 41
```

#### Steps to reproduce the issue

1. Run phpunit

#### What I expected
No error messages or notices.

#### What happened instead
See above.

Run phpunit to reproduce the strict-standards message, then inspect line 41 in tests/php/sync/test_class.jetpack-sync-themes.php where Test_Upgrader_Skin is declared. Update the test so its feedback declaration matches the reported WP_Upgrader_Skin signature, and confirm phpunit finishes without the error or notices.

Written by the indexing model from the issue text.