Reinstate async enumerable tests

These were conditioned on a target we no longer build.

Author: drewnoakes

src/NetMQ.Tests/ClientServer.cs

@@ -82,10 +82,9 @@ public async Task AsyncWithCancellationToken()
             await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await server.ReceiveStringAsync(source.Token));
         }
 
-#if NETCOREAPP3_1
-
+#if NETSTANDARD || NET
         [Fact(Timeout = 120)]
-        public async void AsyncEnumerableCanceled()
+        public async Task AsyncEnumerableCanceled()
         {
             using CancellationTokenSource source = new CancellationTokenSource();
             using var server = new ServerSocket();
@@ -100,7 +99,7 @@ await Assert.ThrowsAnyAsync<OperationCanceledException>(async () =>
         }
 
         [Fact(Timeout = 1000)]
-        public void AsyncEnumerable()
+        public async Task AsyncEnumerable()
         {
             using var server = new ServerSocket();
             int port = server.BindRandomPort("tcp://*");
@@ -152,12 +151,10 @@ public void AsyncEnumerable()
             client.Send("1");
             client.Send("1");
 
-            t1.Wait();
-            t2.Wait();
+            await Task.WhenAll(t1, t2);
 
             Assert.Equal(15002, totalCount);
         }
-
 #endif
     }
 }