Fix fatal in test run under PHP 8

```
Fatal error: Declaration of PostFilters::setUp() must be compatible with Yoast\PHPUnitPolyfills\TestCases\TestCase::setUp(): void in /builds/wp-plugins/view-all-posts-pages/tests/test-post-filters.php on line 40
```

tests/test-post-filters.php

@@ -37,7 +37,7 @@ class PostFilters extends WP_UnitTestCase {
 	/**
 	 * Prepare data for tests.
 	 */
-	public function setUp() {
+	protected function setUp() {
 		parent::setUp();
 
 		static::$post_id = $this->factory->post->create(