CMake: update documentation for `ngen_add_test`

This PR updates the inline documentation for ngen_add_test. The DEPENDS argument wasn't documented.

Checklist

[X] PR has an informative and human-readable title
[X] Changes are limited to a single goal (no scope creep)
[X] Code can be automatically merged (no conflicts)
[X] Code follows project standards (link if applicable)
[X] Passes all existing automated tests
[X] Any change in functionality is tested
[X] New functions are documented (with a description, list of inputs, and expected output)
[X] Placeholder code is flagged / future todos are captured in comments
[X] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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