REST API: Add test class file incorrectly omitted from [45807].

Props @TimothyBlynJacobs.
See #43392.


git-svn-id: https://develop.svn.wordpress.org/trunk@45808 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/class-jsonserializable-object.php

@@ -0,0 +1,21 @@
+<?php
+/**
+ * Unit Tests: JsonSerializable_Object
+ *
+ * @package WordPress
+ * @subpackage UnitTests
+ * @since 5.3.0
+ */
+
+class JsonSerializable_Object implements JsonSerializable {
+
+	private $data;
+
+	public function __construct( $data ) {
+		$this->data = $data;
+	}
+
+	public function jsonSerialize() {
+		return $this->data;
+	}
+}