Migrate BuyNow and CartCheckout buttons to Vitest

[frehner]

Description

Part of #670

Additional context

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Before submitting the PR, please make sure you do the following:

• Read the Contributing Guidelines
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123)
• Update docs in this repository according to your change
• Run yarn changeset add if this PR cause a version bump based on Keep a Changelog and adheres to Semantic Versioning

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run "yarn changeset add" to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.

[mrkldshv]

Do we need to reset mocks after each test? From what I remember this is done automatically now.

[frehner]

You're right, thanks!