test(async): make debounce tests deterministic

[tomas-zijdemans]

Ref: #7022 (comment)

Replace await delay() with synchronous flush()/clear() calls in debounce tests. Real timers can be unreliable in CI, making these tests flaky. Every behaviour under test can be verified deterministically without waiting for timers to fire.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.83%. Comparing base (af9b074) to head (cfe17d6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7034   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files         624      624           
  Lines       49629    49629           
  Branches     8721     8721           
=======================================
  Hits        46571    46571           
  Misses       2478     2478           
  Partials      580      580           

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[kt3k]

LGTM. Thanks!