Update data-tests.md (clarify usage of singular tests in the tests directory)

[diegomorales-dbtlabs]

clarify usage of singular tests in the tests directory

Added a note to explain that singular tests in the tests directory are automatically executed during dbt test and should not be referenced in schema.yml to avoid errors.

as explained in this post
https://discourse.getdbt.com/t/custom-test-not-recognized/12036

What are you changing in this pull request and why?

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."
• The content in this PR requires a dbt release note, so I added one to the release notes page.

---

🚀 Deployment available! Here are the direct links to the updated files:

• https://docs-getdbt-com-git-diegomorales-dbtlabs-patch-1-dbt-labs.vercel.app/docs/build/data-tests

[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. 😄

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated (UTC)
docs-getdbt-com	✅ Ready (Inspect)	Visit Preview	Jan 15, 2025 11:44pm

[matthewshaver]

Thank you @diegomorales-dbtlabs