test: cleanup testConfig.baseRoute and use testConfig.previewBase

[sapphi-red]

Description

1. Remove testConfig.baseRoute as it was not used
   • it was introduced by fix: objurl for type module, and concurrent tests #8541 and then removed by test: use preview server #9992
2. Fix relative-base test to run the preview server with a different base
   • testConfig.baseRoute was introduced to do this. So I restored that by changing the base when running preview.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

I think I'm somewhat leaning to the approach we have before, maybe renaming as testConfig.previewBase would make more sense? The custom plugin to set the base only for preview is a bit complex to setup.

[sapphi-red]

There was a problem with the previous testConfig.baseRoute that it didn't handle pnpm run preview.
This was the reason I wrote this way but I can add it later when #12298 is merged if it's better.

[bluwy]

Yeah if #12298 is merged first to simplify the changes, then I'd be fine with it 👍

[sapphi-red]

Renamed to previewBase and made it test-only.

[sapphi-red]

This file is not using relative path, so this can be removed.

[bluwy]

Awesome! Thanks for making the changes