test: fix properly use terser for preload test

[sapphi-red]

Description

build.terserOptions was set for #4163 but build.minify was not set.
This PR simply adds minify: 'terser'.

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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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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• Ideally, include relevant tests that fail without this PR but pass with it.