[8.x] Fix checkout form test to properly resolve nested tags (#1272)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: duncanmcclean

tests/Tags/CheckoutTagTest.php

@@ -38,20 +38,18 @@
 test('can output checkout form', function () {
     fakeCartWithLineItem();
 
-    $this->tag->setParameters([]);
-
-    $this->tag->setContent('
-        <h2>Checkout</h2>
-
-        {{ sc:gateways }}
-            ---
-            {{ name }} - Duncan Cool ({{ config:is-duncan-cool }}) - Haggis - Tatties
-            ---
-        {{ /sc:gateways }}
+    $usage = (string) tag('
+        {{ sc:checkout }}
+            <h2>Checkout</h2>
+
+            {{ sc:gateways }}
+                ---
+                {{ name }} - Duncan Cool ({{ config:is-duncan-cool }}) - Haggis - Tatties
+                ---
+            {{ /sc:gateways }}
+        {{ /sc:checkout }}
     ');
 
-    $usage = $this->tag->index();
-
     expect($usage)->toContain('Test On-site Gateway - Duncan Cool (yes) - Haggis - Tatties');
     expect($usage)->toContain('<form method="POST" action="http://localhost/!/simple-commerce/checkout"');
 });