GH-1126 Fixed default app port in Tests

Ensured AggregateApplicationTests, AggregateWithMainTest and AggregationTest arn't using default 8080 port

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/aggregate/main/AggregateWithMainTest.java

@@ -18,6 +18,7 @@ package org.springframework.cloud.stream.test.aggregate.main;
 
 import java.util.concurrent.TimeUnit;
 
+import org.junit.Before;
 import org.junit.Test;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -42,6 +43,11 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 public class AggregateWithMainTest {
 
+	@Before
+	public void before() {
+		System.setProperty("server.port", "0");
+	}
+
 	@SuppressWarnings("unchecked")
 	@Test
 	public void testAggregateApplication() throws InterruptedException {