Use integration testing with auditing code in runner specs
What does this MR do and why?
This MR changes the EE tests to use integration testing when performing operations on runners, to expose any bugs in auditing code. This is a corrective action for 2 recent incidents (gitlab-com/gl-infra/production#18984 (closed) and the related https://gitlab.com/gitlab-com/gl-infra/production/-/issues/18994). The approach taken here corresponds to #509082 (closed).
For context, the original problem was that the Gitlab::Audit::CiRunnerTokenAuthor class was missing the #impersonated? method, so tracking that event caused a NoMethodError.
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