fix: ensure test layer is in the build DAG

[LucasRoesler]

Some builders will exclude layers that do not contribute to the final target layer, meaning the ship layer for these templates. This means that the test layer is often optimized away by modern builders.

We add the test layer as a dependency of the ship layer to ensure it is always run.

Motivation and Context

• [x] I have raised an issue to propose this change (required)
• [x] My issue has received approval from the maintainers or lead with the design/approved label

Resolves #71

How Has This Been Tested?

Yes, running docker build will now show the test layer in the output. This was also documented in the issue.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.