migrate Grpc.Examples.Tests to NUnit3

src/csharp/Grpc.Examples.Tests/MathClientServerTests.cs

@@ -149,7 +149,7 @@ namespace Math.Tests
             using (var call = client.Fib(new FibArgs { Limit = 0 }, 
                 deadline: DateTime.UtcNow.AddMilliseconds(500)))
             {
-                var ex = Assert.Throws<RpcException>(async () => await call.ResponseStream.ToListAsync());
+                var ex = Assert.ThrowsAsync<RpcException>(async () => await call.ResponseStream.ToListAsync());
 
                 // We can't guarantee the status code always DeadlineExceeded. See issue #2685.
                 Assert.Contains(ex.Status.StatusCode, new[] { StatusCode.DeadlineExceeded, StatusCode.Internal });