Use fixed date to fix flaky unit test of StoreStatsChartViewModel

[itsmeichigo]

Description

As the title suggests, this PR aims to fix flaky tests for StoreStatsChartViewModel due to the use of Date(). The fix is to use a fixed date to avoid failed tests.

Testing instructions

CI passing is sufficient

Screenshots

N/A

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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 by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr12594-fc36797
	Version	18.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	fc36797
	App Center Build	WooCommerce - Prototype Builds #8876

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

[selanthiraiyan]

Thanks for fixing this. ❤️