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/class-test-case.php

@@ -8,6 +8,8 @@
 
 namespace View_All_Posts_Pages\Tests;
 
+use WP_UnitTestCase;
+
 if ( version_compare( phpversion(), '8.0.0', '<' ) ) {
 	/**
 	 * Class TestCase.