update to use new nuget client in tests

src/csharp/.nuget/packages.config

src/csharp/Grpc.Core.Tests/packages.config

src/csharp/Grpc.Core.Tests/project.json

src/csharp/Grpc.Examples.MathClient/packages.config

src/csharp/Grpc.Examples.MathServer/packages.config

src/csharp/Grpc.IntegrationTesting.QpsWorker/packages.config

...harp/Grpc.IntegrationTesting.StressClient/packages.config

templates/src/csharp/Grpc.Core.Tests/project.json.template

tools/run_tests/pre_build_csharp.bat

tools/run_tests/pre_build_csharp.sh