Fix flaky tests (elastic#175965)

## Summary

These changes fix flaky alert assignments cypress tests:
- elastic#173429
- elastic#172520
- elastic#172557

@MadameSheema mentioned that there were a lot of changes around login
functionality recently which could have fixed original issues. We agreed
that we will run a flaky test runner and if everything works fine we
would un-skip tests and monitor them after the merge.

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ESS 97
times](https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/5064)
- [Serverless 97
times](https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/5052)

x-pack/test/security_solution_cypress/cypress/e2e/detection_response/detection_engine/detection_alerts/assignments/assignments.cy.ts

@@ -42,8 +42,7 @@ import {
 } from '../../../../../tasks/alert_assignments';
 import { ALERTS_COUNT } from '../../../../../screens/alerts';
 
-// Failing: See https://github.com/elastic/kibana/issues/173429
-describe.skip('Alert user assignment - ESS & Serverless', { tags: ['@ess', '@serverless'] }, () => {
+describe('Alert user assignment - ESS & Serverless', { tags: ['@ess', '@serverless'] }, () => {
   before(() => {
     cy.task('esArchiverLoad', { archiveName: 'auditbeat_multiple' });
 

x-pack/test/security_solution_cypress/cypress/e2e/detection_response/detection_engine/detection_alerts/assignments/assignments_serverless_complete.cy.ts

@@ -20,8 +20,7 @@ import {
   loadPageAs,
 } from '../../../../../tasks/alert_assignments';
 
-// FLAKY: https://github.com/elastic/kibana/issues/172557
-describe.skip(
+describe(
   'Alert user assignment - Serverless Complete',
   {
     tags: ['@serverless'],

x-pack/test/security_solution_cypress/cypress/e2e/detection_response/detection_engine/detection_alerts/assignments/assignments_serverless_essentials.cy.ts

@@ -20,8 +20,7 @@ import {
   loadPageAs,
 } from '../../../../../tasks/alert_assignments';
 
-// FLAKY: https://github.com/elastic/kibana/issues/172520
-describe.skip(
+describe(
   'Alert user assignment - Serverless Essentials',
   {
     tags: ['@serverless'],