Add custom role spec for Mutation.vulnerabilitiesDismiss

What does this MR do and why?

In %17.0 the Developer role will no longer have the admin_vulnerability permission. To make sure that we're able to roll out this deprecation in a safe way this MR adds a spec to ensure that a user that belongs to a custom role with the :admin_vulnerability permission is able to execute the vulnerabilitiesDismiss mutation.

#403153 (closed)

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