Disable arkose_labs_signup_challenge feature flag in specs

This is a corrective action for #391231 (closed)

What does this MR do and why?

This MR reverts the removal of stub_feature_flags(arkose_labs_signup_challenge: false) in !111156 (merged).

Stub the feature flag as disabled instead of relying on absence of application setting values as well as ARKOSE_LABS_PUBLIC_KEY and ARKOSE_LABS_PRIVATE_KEY env vars to disable the feature.

This is ideal since the presence of env vars can vary between project CI env settings (e.g. gitlab-org/gitlab vs gitlab-org/security/gitlab) which would make the spec unreliable.

See #391231 (closed) and #389523 (closed) for previous related master:broken issues.

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