test: make Brotli 16GB test wait for backpressure

[trivikr]

This updates parallel/test-zlib-brotli-16GB to wait until the Brotli
decoder reaches the expected readable-buffer backpressure point
before asserting the buffered chunk count.

The previous test used a fixed timeout and could check _readableState.buffer.length
before libuv worker-pool processing had pushed any output, causing intermittent
failures like 0 !== 4 on loaded CI machines.

Refs: https://github.com/nodejs/reliability/blob/main/reports/2026-05-17.md#jstest-failure

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Assisted-by: openai:gpt-5.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (4639dcb) to head (143affe).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63389      +/-   ##
==========================================
+ Coverage   90.29%   90.31%   +0.02%     
==========================================
  Files         730      730              
  Lines      234695   234695              
  Branches    43956    43957       +1     
==========================================
+ Hits       211927   211975      +48     
+ Misses      14494    14436      -58     
- Partials     8274     8284      +10     

see 47 files with indirect coverage changes

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