Update $effect and testing docs to mention issues with push on state array

[raythurnvoid]

Should prevent misunderstandings like #16092

This PR clarifies in the testing documentation that testing examples should not use $state for arrays that are mutated. It also adds a separate untrack section in the $effect documentation to warn users about updating $state values while depending on those same values within an effect.

This should help clarify common misunderstandings users have when dealing with $state versus regular variables.

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16247-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16247

[Rich-Harris]

Thank you. I'm averse to having 'side-quests' in the docs, so I got to thinking about how we can make this situation more self-explanatory, and #16405 is what I came up with — it tightens up the error message to make it clear that it's happening because an effect reads and writes the same piece of state, logs the site of that state change to eliminate any guesswork, and fleshes out the docs page that the error message links to. As such I'll close this