Fix flaky feature spec "user views incident"

What does this MR do and why?

This MR fixes a potentially flaky spec spec/features/incidents/user_views_incident_spec.rb which tends to pass on CI (after retrying) but fails locally.

Example job: https://gitlab.com/gitlab-org/gitlab/-/jobs/3252358610

It also adds a test case to ensure that Guest authors do not see the Incident actions menu while Guest non-authors do to mirror the current behavior.

See #381421 (closed) for more info.

Screenshots

Guest is author	Guest is not author

How to set up and validate locally

bin/rspec ./spec/features/incidents/user_views_incident_spec.rb

bin/rspec ./spec/features/incidents/user_views_incident_spec.rb:45

MR acceptance checklist

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