Need tests for BuildManifestUtil

[missymessa]

BuildManifestUtil was modified during the Post Build Signing project. It needs to have unit tests to validate the changes that were made.

Specifically, these methods need testing:

• CreateModelFromItems
• CreateSigningInformationModelFromItems
• CreateModel
• ManifestBuildDataHasLocationInformation
• All of SigningInformationModel
• All of BuildModel (may be tested as a result of other functionality being tested, but listing it out here as changes were made to it and it's a small class)

Related PRs:

• Merge signing & asset manifests into a single manifest #5517

Code that needs unit tests:

• https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Build.Tasks.Feed/src/BuildManifestUtil.cs
• https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.VersionTools/lib/src/BuildManifest/Model/SigningInformationModel.cs
• https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.VersionTools/lib/src/BuildManifest/Model/BuildModel.cs

[mmitche]

ETA EOD 11/11

[mmitche]

The build model itself already has a bunch of tests written by @jonfortescue, and I covered it a bunch more here #6550.

This PR covers the rest of the items: #6550