Add test stub

Author: markstory

tests/test_app/Mailer/Preview/Invalid.php

@@ -0,0 +1,15 @@
+<?php
+declare(strict_types=1);
+
+namespace DebugKit\TestApp\Mailer\Preview;
+
+/**
+ * Stub class that is not a valid mailer preview.
+ */
+class Invalid
+{
+    public function hello(): string
+    {
+        return 'hello';
+    }
+}