Restructure test assets and baselines

[ellahathaway]

Related to https://github.com/dotnet/installer/pull/19434#discussion_r1586857580

This PR introduces changes to the file structure of the assets and baselines. The changes group the files by category/test so that the automation of PRs based on updated assets can be done in a smoother way.

The new file structure is as follows:

src/SourceBuild/content/test/Microsoft.DotNet.SourceBuild.SmokeTests/assets/
├── ArtifactsSizeTests
│   └── centos.9-x64.txt
├── DotNetFormatTests
│   ├── FormatTestFormatted.cs
│   └── FormatTestUnformatted.cs
├── local.NuGet.Config
├── online.NuGet.Config
├── LicenseScanTests
│   ├── LicenseExclusions.txt
│   ├── Licenses.runtime.json
│   └── Licenses.source-build-externals.json
├── PoisonTests
│   └── PoisonUsage.txt
└── SdkContentTests
    ├── MsftToSbSdkAssemblyVersions.diff
    ├── MsftToSbSdkFiles.diff
    ├── SdkAssemblyVersionDiffExclusions.txt
    └── SdkFileDiffExclusions.txt

5 directories, 13 files