Implement ArrayAccess on TestResponse (#30817)

When building / testing and JSON API, I like to sometimes interact with the raw JSON payload returned in the response. Normally you would have to access this via $response->original['foo']; however, with this change I implement ArrayAccess on both JsonResponse and TestResponse in order to be able to proxy directly into the response JSON without going through the original property.

src/Illuminate/Foundation/Testing/TestResponse.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Foundation\Testing;
 
+use ArrayAccess;
 use Closure;
 use Illuminate\Contracts\View\View;
 use Illuminate\Database\Eloquent\Model;
@@ -12,12 +13,13 @@
 use Illuminate\Support\Str;
 use Illuminate\Support\Traits\Macroable;
 use Illuminate\Support\Traits\Tappable;
+use LogicException;
 use Symfony\Component\HttpFoundation\StreamedResponse;
 
 /**
  * @mixin \Illuminate\Http\Response
  */
-class TestResponse
+class TestResponse implements ArrayAccess
 {
     use Tappable, Macroable {
         __call as macroCall;
@@ -1210,6 +1212,51 @@ public function __isset($key)
         return isset($this->baseResponse->{$key});
     }
 
+    /**
+     * Determine if the given offset exists.
+     *
+     * @param  string  $offset
+     * @return bool
+     */
+    public function offsetExists($offset)
+    {
+        return isset($this->json()[$offset]);
+    }
+
+    /**
+     * Get the value for a given offset.
+     *
+     * @param  string  $offset
+     * @return mixed
+     */
+    public function offsetGet($offset)
+    {
+        return $this->json()[$offset];
+    }
+
+    /**
+     * Set the value at the given offset.
+     *
+     * @param  string  $offset
+     * @param  mixed  $value
+     * @return void
+     */
+    public function offsetSet($offset, $value)
+    {
+        throw new LogicException('Response data may not be mutated using array access.');
+    }
+
+    /**
+     * Unset the value at the given offset.
+     *
+     * @param  string  $offset
+     * @return void
+     */
+    public function offsetUnset($offset)
+    {
+        throw new LogicException('Response data may not be mutated using array access.');
+    }
+
     /**
      * Handle dynamic calls into macros or pass missing methods to the base response.
      *