Update features/scaffold-theme-tests.feature

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

Author: swissspidy

features/scaffold-theme-tests.feature

@@ -215,7 +215,7 @@ Feature: Scaffold theme unit tests
     When I try `wp scaffold theme-tests ../`
     Then STDERR should be:
       """
-      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters, underscores, and dashes.
       """
     And the return code should be 1