test(decompress): wait for highWaterMark via readable instead of setImmediate loop

[kjsik11]

This relates to...

• test/interceptors/decompress.js (added in fix: honor backpressure in decompression interceptor #5829)
• Fails intermittently in CI since it was merged
  • main (bfea0201): https://github.com/nodejs/undici/actions/runs/35103603339/job/104818987490
  • fix(deduplicate): make terminal callbacks exception-safe #5838: https://github.com/nodejs/undici/actions/runs/35228106751/job/105224759265
  • fix(h2): keep the session ref'd while requests are queued #5839: https://github.com/nodejs/undici/actions/runs/35308594518/job/105485760782

Rationale

The test waits for the body to reach highWaterMark with a loop of 100 setImmediate ticks, which is unreliable when the threadpool is busy.

Changes

Wait for readableHighWaterMark to be reached via the readable event instead of a setImmediate loop.

Tests

• The CI failing on main (bfea0201, before this change): https://github.com/nodejs/undici/actions/runs/35103603339/job/104818987490#step:18:301
• The same job passing with this change : https://github.com/nodejs/undici/actions/runs/35330916420/job/105554808727

Assisted-by: Claude Code

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• [S] Benchmarked (optional)
• [S] Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.53%. Comparing base (bfea020) to head (c0f3dc8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5841   +/-   ##
=======================================
  Coverage   93.53%   93.53%           
=======================================
  Files         110      110           
  Lines       39773    39773           
=======================================
+ Hits        37200    37201    +1     
+ Misses       2573     2572    -1     

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[mcollina]

lgtm