Add tests

tests/PHPUnit/CustomAffixes/FolderWithAn@/ExampleTest.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+class MyCustomClassTest extends PHPUnit\Framework\TestCase
+{
+    public function assertTrueIsTrue()
+    {
+        $this->assertTrue(true);
+    }
+}
+
+uses(MyCustomClassTest::class);
+
+test('custom traits can be used', function () {
+    $this->assertTrueIsTrue();
+});
+
+test('trait applied in this file')->assertTrueIsTrue();