docs(kafka-streams-demo): fix node:test command + include subscribe helper

Reviewer blocker. Documented `node --test demo/kafka-streams/test/`
fails with MODULE_NOT_FOUND because node:test does not accept a bare
directory as a positional spec. Use a glob (`test/*.mjs`) so all four
helper test files are picked up, and add lib/subscribe.mjs to the
helpers list now that it ships alongside the originals.

Author: Zvonimir

demo/kafka-streams/README.md

@@ -75,11 +75,11 @@ between steps.
 ## Tests
 
 The pure helpers (`lib/assertions.mjs`, `lib/poll.mjs`,
-`lib/daemon-auth.mjs`) have `node:test` coverage under
-`demo/kafka-streams/test/`:
+`lib/daemon-auth.mjs`, `lib/subscribe.mjs`) have `node:test` coverage
+under `demo/kafka-streams/test/`:
 
 ```sh
-node --test demo/kafka-streams/test/
+node --test demo/kafka-streams/test/*.mjs
 ```
 
 The full runtime gate (`run.mjs`) is intentionally an