Add Behat test to verify no HTML output during core update

Add test scenario that verifies async translation updates don't output HTML tags during WordPress core updates. The test checks that the output doesn't contain <p>, <div>, <script>, or closing </div> tags.

Co-authored-by: swissspidy <[email protected]>

Author: Copilot

features/core-update.feature

@@ -384,3 +384,24 @@ Feature: Update WordPress core
       """
       Success:
       """
+
+  Scenario: No HTML output from async translation updates during core update
+    Given a WP install
+
+    When I run `wp core update --version=6.2.0 --force`
+    Then STDOUT should not contain:
+      """
+      <p>
+      """
+    And STDOUT should not contain:
+      """
+      <div
+      """
+    And STDOUT should not contain:
+      """
+      <script
+      """
+    And STDOUT should not contain:
+      """
+      </div>
+      """