Fix xUnit1048 warnings

Author: tmatthey

src/NetMQ.Tests/ClientServer.cs

@@ -54,7 +54,7 @@ public void Tcp()
         }
 
         [Fact]
-        public async void Async()
+        public async Task Async()
         {
             using var server = new ServerSocket();
             using var client = new ClientSocket();
@@ -72,7 +72,7 @@ public async void Async()
         }
 
         [Fact]
-        public async void AsyncWithCancellationToken()
+        public async Task AsyncWithCancellationToken()
         {
             using CancellationTokenSource source = new CancellationTokenSource();
             using var server = new ServerSocket();
@@ -85,7 +85,7 @@ public async void AsyncWithCancellationToken()
 #if NETCOREAPP3_1
 
         [Fact(Timeout = 120)]
-        public async void AsyncEnumerableCanceled()
+        public async Task AsyncEnumerableCanceled()
         {
             using CancellationTokenSource source = new CancellationTokenSource();
             using var server = new ServerSocket();

src/NetMQ.Tests/RadioDish.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Threading;
+using System.Threading.Tasks;
 using NetMQ.Sockets;
 using Xunit;
 using Xunit.Abstractions;
@@ -47,7 +48,7 @@ public void TestBlocking()
         }
 
         [Fact]
-        public async void TestAsync()
+        public async Task TestAsync()
         {
             using var radio = new RadioSocket();
             using var dish = new DishSocket();

src/NetMQ.Tests/ScatterGather.cs

@@ -1,4 +1,5 @@
 using System.Threading;
+using System.Threading.Tasks;
 using NetMQ.Sockets;
 using Xunit;
 
@@ -47,7 +48,7 @@ public void TestBlocking()
         }
 
         [Fact]
-        public async void TestAsync()
+        public async Task TestAsync()
         {
             using var scatter = new ScatterSocket();
             using var gather = new GatherSocket();