docs: clarify required build before test

[j-maas]

Description

When I tried running the tests, I got a lot of error messages. It took me a while to figure out that this was because I hadn't built Vite.

Additional context

A previous issue (#4543) has already pointed out this dependency. It was proposed to add a note to the documentation, but if that happened I think it can be made even clearer.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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