Fix Gitlab::InternalEvents::EventDefinitions specs

What does this MR do and why?

The spec was loading events with errors as part of the expectations. Not unsetting the @events variable will make subsequent specs to have the wrong definitions loaded. We need to unset @events if we have changed it in a negative way for testing purposes

Related to gitlab-org/quality/engineering-productivity/master-broken-incidents#2896 (closed)

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