While testing your application, it is highly recommended to exclude Svelvet components as best you can. A library that has a fantastic API for UI-centric testing is @testing-library/svelte.
For example, when rendering the Svelvet component, your test could query for the contents rather than the specific Svelvet instance. Svelvet undergoes frequent version updates, and by not relying on the Svelte component tree you make your test more robust against internal changes in Svelvet.
For SvelteKit apps: While @jest is a reliable option for running your tests, SvelteKit applications are built with Vite and thus require separate configurations. We found that @vitest is a great option for Vite apps as they share the same config files and require little additional work to get started, letting you focus on writing quality tests rather than config issues.