Publish corefx test assets (dotnet/corefx#35473)

Publish corefx test assets (dotnet/corefx#35473)

* Publish corefx test assets

So coreclr and other repos can run the corefx tests.

New step to publish the test zip files to dotnet-core blob storage.

Publishes the build manifest from above to blob storage path:

https://dotnetfeed.blob.core.windows.net/dotnet-core/corefx-tests/$(Version)/$(TargetOS).$(ArchGroup)/$(TargetGroup)/corefx-test-assets.xml

Declare DotNet-Blob-Feed only in official builds

* Code review feedback.

The blob path should NOT be Normalized.

* WIP

Commit migrated from https://github.com/dotnet/corefx/commit/bfb495d39e2d7b5d7cc0b571ed504670b2faa68c