Further simplify integration tests for zipkin-stream

It's not really necessary to use rabbit, but the existing tests
weren't really using the stream components at all because
zipkin spans were being collected by spring-cloud-sleuth-zipkin.

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-stream/src/main/resources/application.yml

@@ -4,3 +4,6 @@ server:
 spring:
   application:
     name: testSleuthApp
+# TODO: remove this once Brave is off the class path
+  zipkin:
+    enabled: false