Remove cmd/testclient directory and references

- Deleted cmd/testclient directory containing test client implementation
- Removed testclient references from Makefile and insecure/Makefile tidy targets

Co-Authored-By: [email protected] <[email protected]>

Author: devin-ai-integration[bot]

Makefile

@@ -238,7 +238,6 @@ tidy:
 	go mod tidy -v
 	cd integration; go mod tidy -v
 	cd crypto; go mod tidy -v
-	cd cmd/testclient; go mod tidy -v
 	cd insecure; go mod tidy -v
 	git diff --exit-code