add link to PR/ticket with mockserver replacement

Author: Tofel

book/src/lib/docker/test_helpers.md

@@ -5,4 +5,6 @@ There two test helper containers:
 * Mockserver
 
 Both represent HTTP mocking solutions, with Mockserver used in k8s and Killgrave outside of it. Since both will soon
-be replaced by a single, in-house solution, their usage is discouraged.
+be replaced by a single, in-house solution, their usage is discouraged.
+
+That worked is begin done in [this PR](https://github.com/smartcontractkit/chainlink-testing-framework/pull/1246) and tracked in [this ticket](https://smartcontract-it.atlassian.net/browse/TT-1608).