Add "when Arkose is down, user can still signup" feature specs

What does this MR do and why?

Resolves #441566 (closed).

Context: In !141750 (merged) feature specs were added to test the scenario when Arkose is down, user can still complete SAML signup flow.

This MR adds coverage for the same scenario on other signup flows—Trial, Subscription, Invite, Standard.

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