test: Attempt to reduce asyncEvent test flakiness

[robdiciuccio]

SUMMARY

Address the test flakiness mentioned in this comment by explicitly clearing the async event polling timeout.

TEST PLAN

FE asyncEvent tests pass without flakiness.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #14497 (aef3009) into master (7182a1b) will decrease coverage by 0.08%.
The diff coverage is 60.00%.

❗ Current head aef3009 differs from pull request most recent head a572e93. Consider uploading reports for the commit a572e93 to get more accurate results

@@            Coverage Diff             @@
##           master   #14497      +/-   ##
==========================================
- Coverage   77.12%   77.03%   -0.09%     
==========================================
  Files         953      953              
  Lines       48189    48191       +2     
  Branches     6051     6052       +1     
==========================================
- Hits        37164    37126      -38     
- Misses      10824    10864      +40     
  Partials      201      201              

Flag	Coverage Δ
javascript	71.93% <60.00%> (-0.01%)	⬇️
presto	?

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Impacted Files	Coverage Δ
...nd/src/explore/components/ExploreViewContainer.jsx	2.33% <ø> (ø)
superset-frontend/src/middleware/asyncEvent.ts	77.61% <60.00%> (-0.42%)	⬇️
superset/db_engine_specs/presto.py	84.42% <0.00%> (-5.48%)	⬇️
superset/connectors/sqla/models.py	88.61% <0.00%> (-1.46%)	⬇️
superset/models/core.py	88.85% <0.00%> (-0.28%)	⬇️

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[ktmud]

LGTM. Thanks for the fix!