disable illegal import detection when running tests

[Rich-Harris]

This is a naive but effective fix for #8180. When running Vitest, both process.env.VITEST and process.env.TEST are true, meaning we can simply ignore illegal imports during unit testing.

I opted for TEST over VITEST because it will then work with any unit testing framework that works with Vite, as long as they adhere to this convention.

I think it's totally fine to skip detection during tests, since it'd be vanishingly rare that something was explicitly testing for that behaviour. The error will show up soon enough in dev, and the build will fail.

One gotcha is that if you do TEST=true npm run dev it will disable illegal import detection during dev (not build), which I think is probably a fine trade-off.

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[Conduitry]

I think we should list this in the docs somewhere. Unless this TEST=true convention is very widespread and somehow everyone's heard about it but me?

[Rich-Harris]

added some docs