[POS][Local Catalog] Use SwiftTesting in SiteSettingsStoreMethodsTests

[joshheald]

Part of: WOOMOB-1252

Description

This PR updates SiteSpecificAppSettingsStoreMethodsTests to use SwiftTesting, in preparation for new tests to be added in WOOMOB-1252 for storing the last sync date.

Steps to reproduce

Check tests pass locally and in CI

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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.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16097-b376b93
	Version	23.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	b376b93
	Installation URL	5ckbb23rep4no

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[jaclync]

Thanks for upgrading the tests, including the outdated expectation! Passed locally.