Fix test: assert vendor/autoload.php does not exist instead of checking contents

Agent-Logs-Url: https://github.com/wp-cli/package-command/sessions/7bc70ad5-73a0-45a9-b337-7be724bfffcb

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/package-install.feature

@@ -1437,10 +1437,7 @@ Feature: Install WP-CLI packages
       Error: Invalid package name '..':
       """
     And STDOUT should be empty
-    And the {PACKAGE_PATH}vendor/autoload.php file should not contain:
-      """
-      COMPROMISED
-      """
+    And the {PACKAGE_PATH}vendor/autoload.php file should not exist
     And the {PACKAGE_PATH}local directory should not exist
 
   @github-api