Fix flaky callout dismissal specs

What does this MR do and why?

Wait for JS events to be setup before dismissing the callout.

This fixes various specs that are flaky.

Example failure: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/11669976890

References

• gitlab-org/quality/test-governance/request-for-help#64
• [Test] spec/features/dashboard/activity_spec.rb... (#566482 - closed)
• [Test] ee/spec/features/dashboards/projects_spe... (#512634 - closed)
• [Test] spec/features/dashboard/issues_spec.rb |... (#560529 - closed)
• [Test] spec/features/dashboard/group_spec.rb | ... (#535235 - closed)
• [Test] ee/spec/features/dashboards/todos_spec.r... (#512781 - closed)
• [Test] spec/features/dashboard/projects_spec.rb... (#535203 - closed)
• [Test] spec/features/dashboard/todos/todos_spec... (#561050 - closed)
• [Test] ee/spec/features/dashboards/activity_spe... (#512637 - closed)

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