[System] Fix ClientWebSocketTest to not throw exceptions in the constructor, but instead in the individual tests. Fixes #47167. (mono#4180)

Finding a free port on watchOS fails with a PlatformNotSupportedException, but if the exception is thrown in the constructor, the ExpectedException attribute isn't taken into account, causing the test to error out. https://bugzilla.xamarin.com/show_bug.cgi?id=47167

Author: rolfbjarne

mcs/class/System/Test/System.Net.WebSockets/ClientWebSocketTest.cs

@@ -17,7 +17,14 @@ namespace MonoTests.System.Net.WebSockets
 public class ClientWebSocketTest
 {
 const string EchoServerUrl = "ws://corefx-net.cloudapp.net/WebSocket/EchoWebSocket.ashx";
-int Port = NetworkHelpers.FindFreePort ();
+int port;
+int Port {
+get {
+if (port == 0)
+port = NetworkHelpers.FindFreePort ();
+return port;
+}
+}
 HttpListener _listener;
 HttpListener listener {
 get {
@@ -158,7 +165,12 @@ public void CloseAsyncTest ()
 Assert.AreEqual (WebSocketState.Closed, socket.State);
 }
 
-[Test, ExpectedException (typeof (InvalidOperationException))]
+[Test]
+#if FEATURE_NO_BSD_SOCKETS
+[ExpectedException (typeof (PlatformNotSupportedException))]
+#else
+[ExpectedException (typeof (InvalidOperationException))]
+#endif
 public void SendAsyncArgTest_NotConnected ()
 {
 socket.SendAsync (new ArraySegment<byte> (new byte[0]), WebSocketMessageType.Text, true, CancellationToken.None);
@@ -172,7 +184,11 @@ public void SendAsyncArgTest_NoArray ()
 socket.SendAsync (new ArraySegment<byte> (), WebSocketMessageType.Text, true, CancellationToken.None);
 }
 
-[Test, ExpectedException (typeof (InvalidOperationException))]
+#if FEATURE_NO_BSD_SOCKETS
+[ExpectedException (typeof (PlatformNotSupportedException))]
+#else
+[ExpectedException (typeof (InvalidOperationException))]
+#endif
 public void ReceiveAsyncArgTest_NotConnected ()
 {
 socket.ReceiveAsync (new ArraySegment<byte> (new byte[0]), CancellationToken.None);