Add unit test that checks for unused Dockerfiles

### Describe the Problem

Sometimes, we accidentally leave unused Dockerfiles in the `src/` folder that are no longer built or published.

### Describe the Solution

There should be a test, in [GeneratedArtifactTests.cs](https://github.com/dotnet/dotnet-docker/blob/main/tests/Microsoft.DotNet.Docker.Tests/GeneratedArtifactTests.cs), which fails if there are any Dockerfiles in `src/` that are not referenced by `manifest.json`.