Improve async support in Spring MVC Test

When obtaining an async result, tests will now await concurrent
processing to complete for the exact amount of time equal to the
actual async timeout value.

Issue: SPR-9875

spring-test-mvc/src/test/java/org/springframework/test/web/servlet/StubMvcResult.java

@@ -120,4 +120,8 @@ public class StubMvcResult implements MvcResult {
 		this.response = response;
 	}
 
+	public Object getAsyncResult() {
+		return null;
+	}
+
 }