generate/identitytests: Correct PlanCheck for single identity attribute when plannableImportAction="Replace"

[ewbankkit]

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If a change needs to be reverted, we will publish an updated version of the library.

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Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

Noticed while adding RI to Timestream InfluxDB (#47052). If an import step results in resource replacement then the identity attribute will plan to Unknown.
Restricted to the single identity attribute case as for many multi-attribute parameterized identity resource, not all identity attributes will be unknown.

[github-actions]

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[jar-b]

LGTM 🚀

[github-actions]

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