Remove finicky exception message assertion

In SimpleNioTransportTests we assert that an IOException has a certain
message. This message appears that it is not dependible (and might
change based on platform).

Our other transport tests (mock and netty) do not make this assertion.
Instead they only assert on our application exception message. This
commit removes the IOException message assertion. And retains the
ConnectTransportException message assertion.

test/framework/src/test/java/org/elasticsearch/transport/nio/SimpleNioTransportTests.java

@@ -104,7 +104,6 @@ public class SimpleNioTransportTests extends AbstractSimpleTransportTestCase {
             assertThat(e.getMessage(), containsString("[127.0.0.1:9876]"));
             Throwable cause = e.getCause();
             assertThat(cause, instanceOf(IOException.class));
-            assertEquals("Connection refused", cause.getMessage());
         }
     }