Add unit tests for AccountCreationFormViewModel

[jaclync]

Part of #7891

Description

To meet the timeline (code freeze today), I deferred the unit tests until after the feature was complete. Debounce duration was DI'ed with 0 in the unit tests so as not to lengthen the testing time.

Testing instructions

CI should be sufficient, but feel free to test the account creation form to make sure the debounce still works.

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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[wpmobilebot]

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[selanthiraiyan]

LGTM 🚢

The unit tests are running successfully locally. To fix CI, I have restarted the buildkite unit tests script. 🤞