INT-1884 Fix Intermittent Test Failure

The test is verifying at least one more poll is executed after capturing
the initial poll count. However we only did one receive and relying on
luck that another poll happened in the interim. Changed to do 2 receives.

spring-integration-jmx/src/test/java/org/springframework/integration/monitor/MessageSourceMonitoringIntegrationTests.java

@@ -55,6 +55,7 @@ public class MessageSourceMonitoringIntegrationTests {
 
 			int before = service.getCounter();
 			channel.receive(1000L);
+			channel.receive(1000L);
 			assertTrue(before < service.getCounter());
 
 			int count = exporter.getSourceMessageCount(monitor);