update nuget dependencies to nunit3

src/csharp/.nuget/packages.config

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

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

src/csharp/Grpc.Examples.Tests/Grpc.Examples.Tests.csproj

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

src/csharp/Grpc.Examples/Grpc.Examples.csproj

src/csharp/Grpc.Examples/packages.config

...harp/Grpc.HealthCheck.Tests/Grpc.HealthCheck.Tests.csproj

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

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/Grpc.IntegrationTesting/packages.config