Tests: Disable NIO transport mechanism in tests

Due to test instability the new transport mechanism is
always disabled and does not randomly pick the new IO
transport.

test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java

@@ -892,7 +892,7 @@ public abstract class ESTestCase extends LuceneTestCase {
 
     @BeforeClass
     public static void setUseNio() throws Exception {
-        useNio = randomBoolean();
+        useNio = false;
     }
 
     public static String getTestTransportType() {