Use static calls for assertions in tests

phpcs-errors:226 phpunit-status:successful phpstan-errors:202

tests/startPageTest.php

@@ -17,7 +17,7 @@ final class StartPageTest extends TestCase {
         include __DIR__ . "/../index.php";
         $output = $this->getActualOutput();
 
-        $this->assertEquals(
+        self::assertEquals(
             true,
             is_string($output)
         );