Add configuration caveats section to unit-tests.md

[jairus-m]

Proposing the addition of a small section on "Caveats to unit test configuration" that highlights proper configuration of mock inputs for data that isn't part of the core unit test logic.

What are you changing in this pull request and why?

• A write-up with more detail (which also catalysed this proposal) can be found here.

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• I have added checklist item(s) to this list for anything anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

[welcome]

Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
We use Markdown and some HTML to write the dbt product documentation. When writing content, you can use our style guide and content types to understand our writing standards and how we organize information in the dbt product docs.
We'll review your contribution and respond as soon as we can. 😄

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