403 error when to create node from one to another Drupal instance using rest service under testscript

Hi,

I am getting 403 error code response while creating node from one site to another through functional test case. Below are the steps:

1. Setup a multi-site that have single codebase with multiple databases e.g Site A and Site B.
2. Enabled POST and PATCH rest service under entity:node on Site B.
3. Created a test script which extend WebTestBase under custom module into Site A and run it. Below is the snippet

4. public function testSimpleNodeSync() {
        $admin_user = $this->drupalCreateUser(array('administer users'));
       $this->drupalLogin($admin_user);
       $node = Node::create(['type' => 'page', 'title' => 'test', 'uid' => 1]);
       $node->save();
       $filtered_node_json = $this->filterNodeFields($node);
       $client = \Drupal::httpClient();
       try {
       $response = $client->post('http://site-b.com/entity/node', [
         'auth' => ['admin', 'admin123'],
         'headers' => [
           'Content-Type' => 'application/json',
           'Accept' => 'application/json',
         ],
         'body' => $filtered_node_json,
       ]);
       }
       catch (\Exception $e) {
         $this->fail($e->getMessage());
       }
      }

5. It always fall under catch clause and gives 403 error.