test(websocket): fix flaky send test

[mcollina]

Description

• refactor test/websocket/send.js to use async/await with once(...)
• register deterministic cleanup for every websocket client/server pair
• wait for close events instead of resolving tests before teardown finishes

Fixes: #5181

Testing

• 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 (da0eebc) to head (16a6a4b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5232   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         110      110           
  Lines       36353    36353           
=======================================
+ Hits        33907    33909    +2     
+ Misses       2446     2444    -2     

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