test(websocket): fully deflake send test

[mcollina]

Description

• finish deflaking test/websocket/send.js
• await open, message, and close events deterministically with once(...)
• register cleanup that waits for websocket client/server teardown before test exit
• avoid the close-event race in Sending data after close by subscribing before closing

Follow-up to #5232
Related: #5181

Testing

• npx borp -p "test/websocket/send.js"
• repeated 50x npx borp -p "test/websocket/send.js"
• npm run test:websocket
• npm run lint

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (e58fff7) to head (871481e).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5236   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         110      110           
  Lines       36353    36366   +13     
=======================================
+ Hits        33909    33922   +13     
  Misses       2444     2444           

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