fix: validate package existence in conda build --test

[swap357]

Description

This PR changes the behavior when no tests are found for a package during conda build. Previously, the code would silently succeed by returning True when no tests were found. This could mask issues where tests were accidentally omitted or misconfigured.

Now, it will raise a CondaBuildUserError with a clear message indicating that no tests were found for the package. This makes the behavior more explicit and helps users identify when they may have accidentally omitted test configurations.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

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