test: Avoid APFS integration timeout

[shepilov]

Initial problem

The APFS integration job was timing out in test/integration/differential_sync.js for the scenario where a local folder is created with the same path as an excluded remote folder and the user chooses to create a conflict. The failing test waited for local-end only after helpers.local.side.start() had resolved, but start() already waits for the initial scan to finish. The watcher emits local-end before resolving that start() promise, so the test could attach its listener too late and then wait forever until Mocha hit the 20s timeout.

The same race also existed after creating Photos/ and Photos/My Image.png: the listener was registered after the filesystem writes, so a fast watcher could flush the events before the test started waiting.

Changes

• Remove the redundant local-end wait after helpers.local.side.start().
• Use a one-shot once('local-end', ...) listener.
• Register the local watcher completion promise before creating the local folder and file.

Validation

• ./node_modules/.bin/eslint test/integration/differential_sync.js
• git diff --check origin/master...HEAD

The targeted APFS integration test was not run locally because this checkout has no .env.test, and no local Cozy stack is running on cozy.localhost:8080 / 127.0.0.1:8080.