Update src/aks-preview/azext_aks_preview/tests/latest/test_aks_commands.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: m-chau

src/aks-preview/azext_aks_preview/tests/latest/test_aks_commands.py

@@ -21498,7 +21498,9 @@ def test_aks_upgrade_with_tier_switch(self, resource_group, resource_group_locat
             "aks upgrade --resource-group={resource_group} --name={name} "
             "--tier standard -k {upgrade_k8s_version} --yes"
         )
-        self.cmd(fail_upgrade_cmd, expect_failure=True)
+        fail_upgrade_result = self.cmd(fail_upgrade_cmd, expect_failure=True)
+        self.assertIn("AKSLongTermSupport", fail_upgrade_result.error)
+        self.assertIn("Premium", fail_upgrade_result.error)
 
         upgrade_cmd = (
             "aks upgrade --resource-group={resource_group} --name={name} "