[fix flaky on mki] add dismiss all toasts (elastic#184659)

## Summary

resolves: elastic#184655

Ran this from my local, against an MKI oblt cluster.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/test_serverless/functional/test_suites/observability/cases/list_view.ts

@@ -17,6 +17,7 @@ export default ({ getPageObject, getService }: FtrProviderContext) => {
   const svlCommonNavigation = getPageObject('svlCommonNavigation');
   const svlCommonPage = getPageObject('svlCommonPage');
   const svlObltNavigation = getService('svlObltNavigation');
+  const toasts = getService('toasts');
 
   describe('Cases list', function () {
     before(async () => {
@@ -258,6 +259,7 @@ export default ({ getPageObject, getService }: FtrProviderContext) => {
         createNCasesBeforeDeleteAllAfter(1, getPageObject, getService);
 
         it('deletes a case correctly', async () => {
+          await toasts.dismissAll();
           await cases.casesTable.deleteCase(0);
           await cases.casesTable.waitForTableToFinishLoading();
           await cases.casesTable.validateCasesTableHasNthRows(0);