INT-2839 Fix Test Race Condition

A secondary part of the new gateway test was to ensure we
reused the cached connection. However, there was a race condition
in that the connection may not yet have been returned to the pool
by the reader thread that received the response.

spring-integration-ip/src/test/java/org/springframework/integration/ip/tcp/connection/CachingClientConnectionFactoryTests.java

@@ -320,6 +320,9 @@ public class CachingClientConnectionFactoryTests {
 		assertNotNull(m);
 		assertEquals("foo:" + "Hello, world!", new String((byte[]) m.getPayload()));
 
+		// wait a short time to allow the connection to be returned to the pool
+		Thread.sleep(1000);
+
 		// assert we use the same connection from the pool
 		toGateway.send(new GenericMessage<String>("Hello, world2!"));
 		m = fromGateway.receive(1000);