make ConnectShouldActivateKeepAliveIfSessionIs test less flaky (#1410)

* make ConnectShouldActivateKeepAliveIfSessionIs test less flaky

wait longer and expect at least two instead of exactly two.

example: https://ci.appveyor.com/project/drieseng/ssh-net/builds/49849877/job/9rrtw6j8eu3i5p8o?fullLog=true

* fix net462 build

test/Renci.SshNet.Tests/Classes/BaseClientTest_NotConnected_KeepAliveInterval_NotNegativeOne.cs

@@ -69,11 +69,11 @@ namespace Renci.SshNet.Tests.Classes
 
             _client.Connect();
 
-            // allow keep-alive to be sent twice
-            Thread.Sleep(250);
+            // allow keep-alive to be sent at least twice with some margin for error
+            Thread.Sleep(5 * (int)_keepAliveInterval.TotalMilliseconds);
 
-            // Exactly two keep-alives should be sent
-            SessionMock.Verify(p => p.TrySendMessage(It.IsAny<IgnoreMessage>()), Times.Exactly(2));
+            // At least two keep-alives should be sent
+            SessionMock.Verify(p => p.TrySendMessage(It.IsAny<IgnoreMessage>()), Times.AtLeast(2));
         }
 
         private class MyClient : BaseClient