test: fix playground config variants

[patak-dev]

Description

Our current test setup for config variants loaded the config files from the original playground folder instead of the playground-temp copy. This caused issues when using __dirname to resolve paths. I'm surprised that the tests were working. This is the issue discovered in #13712 in this comment.

It worked by looking for a vite.config.* file in the folder of the playground variant spec.

/playground/assets/__tests__/relative_path/assets-relative-path.spec.js
/playground/assets/__tests__/relative_path/vite.config.js

But the files inside __tests__ are not copied to playground-temp.

Most of these config files were a re-export of the config defined at root.

This PR defines a new convention. For variants, if there is a config named vite.config-{variantName}.js in root, it is then used (these are copied already to playground-temp). We were already using this convention to be able to run the playgrounds outside of Vitest. So we just needed to delete all the vite.config.js files inside __tests__ (the only ones that were different were for worker sourcemap, but I reworked them... it is better because we can now run them out of tests too).

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

I like the new convention 👍