test: fix flaky keepalive timeout test on slow platforms

[Han5991]

Fixes: #60656

The test was setting keepAliveTimeout with platformTimeout() but
keepAliveTimeoutBuffer remained at the hardcoded default of 1000ms.
This caused timing issues on slow CI platforms where platformTimeout
scales up timeout values but the buffer did not scale accordingly.

Now both keepAliveTimeout and keepAliveTimeoutBuffer are set using
platformTimeout() to ensure consistent behavior across all platforms

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.53%. Comparing base (bd3a202) to head (ce00f6a).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60664      +/-   ##
==========================================
- Coverage   88.55%   88.53%   -0.02%     
==========================================
  Files         703      703              
  Lines      208077   208077              
  Branches    40083    40081       -2     
==========================================
- Hits       184254   184221      -33     
- Misses      15841    15876      +35     
+ Partials     7982     7980       -2     

see 38 files with indirect coverage changes

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