Fix unit tests native dependencies

[ThomasGoulet73]

Needed for dotnet/wpf#8215

Description

Fixes unit tests native dependencies that were globbed too early which means that it skipped some dlls that were copied later in the build.

I also added comments for the code I previoulsy added to explain why it is needed.

Customer Impact

None, tests only.

Regression

No.

Testing

Local build + running unit tests.

Risk

None.

Microsoft Reviewers: Open in CodeFlow