Update WebFluxObservationPropagationTests

Add missed `Awaitility.await` import

Author: artembilan

spring-integration-webflux/src/test/java/org/springframework/integration/webflux/observation/WebFluxObservationPropagationTests.java

@@ -56,6 +56,7 @@
 import org.springframework.web.reactive.config.EnableWebFlux;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
 
 /**
  * @author Artem Bilan