Fixes Cypress flake cypress test (#115270)

## Summary

Fixes flake cypress test

Fixes #115245 
See also: #114075, #115245

### Checklist

- [x] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios

x-pack/plugins/security_solution/cypress/screens/alerts.ts

@@ -58,7 +58,7 @@ export const TAKE_ACTION_POPOVER_BTN = '[data-test-subj="selectedShowBulkActions
 
 export const TIMELINE_CONTEXT_MENU_BTN = '[data-test-subj="timeline-context-menu-button"]';
 
-export const ATTACH_ALERT_TO_CASE_BUTTON = '[data-test-subj="attach-alert-to-case-button"]';
+export const ATTACH_ALERT_TO_CASE_BUTTON = '[data-test-subj="add-existing-case-menu-item"]';
 
 export const ALERT_COUNT_TABLE_FIRST_ROW_COUNT =
   '[data-test-subj="alertsCountTable"] tr:nth-child(1) td:nth-child(2) .euiTableCellContent__text';