Add response information to TldFanoutAction

TldFanoutAction now returns an HTTP response detailing the actions it's taking.
The format is as follows:

OK: Launched the following 3 tasks in queue the-queue
- Task: task-a6ad250b-a9d8-427d-bbf7-eb736e6f4dcb, tld: com, endpoint: /the/servlet/com
- Task: task-cf6c4bb4-0542-411e-ae4d-723beec09e9c, tld: net, endpoint: /the/servlet/net
- Task: task-57899661-fc3f-4049-a265-d6604051406e, tld: org, endpoint: /the/servlet/org

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152264954

javatests/google/registry/cron/BUILD

@@ -17,6 +17,8 @@ java_library(
         "//javatests/google/registry/testing",
         "//third_party/java/objectify:objectify-v4_1",
         "@com_google_appengine_api_1_0_sdk//:testonly",
+        "@com_google_appengine_api_stubs",
+        "@com_google_appengine_testing",
         "@com_google_guava",
         "@com_google_truth",
         "@javax_servlet_api",