chore: update stress tests github action

[thisisabhash]

Issue #

Stress test runs have been flaky.
https://github.com/aws-amplify/amplify-swift/actions/runs/19862211955
https://github.com/aws-amplify/amplify-swift/actions/runs/19862202540
https://github.com/aws-amplify/amplify-swift/actions/runs/19841453552
https://github.com/aws-amplify/amplify-swift/actions/runs/19836950121

Description

This PR

• adds a retry on failure for stress tests, similar to integration tests.
• refactors testMultipleSubscriptions().

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.66%. Comparing base (8e6dedf) to head (b1fa94b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4112      +/-   ##
==========================================
+ Coverage   66.62%   66.66%   +0.04%     
==========================================
  Files        1129     1129              
  Lines       42585    42585              
==========================================
+ Hits        28371    28389      +18     
+ Misses      14214    14196      -18     

Flag	Coverage Δ
API_plugin_unit_test	68.38% <ø> (ø)
AWSPluginsCore	68.54% <ø> (ø)
Amplify	46.72% <ø> (ø)
Analytics_plugin_unit_test	84.84% <ø> (ø)
Auth_plugin_unit_test	72.29% <ø> (-0.02%)	⬇️
DataStore_plugin_unit_test	81.97% <ø> (+0.28%)	⬆️
Geo_plugin_unit_test	73.39% <ø> (ø)
Logging_plugin_unit_test	64.86% <ø> (ø)
Predictions_plugin_unit_test	33.89% <ø> (ø)
PushNotifications_plugin_unit_test	87.41% <ø> (ø)
Storage_plugin_unit_test	77.68% <ø> (-0.03%)	⬇️
unit_tests	66.66% <ø> (+0.04%)	⬆️

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[thisisabhash]

Test run: https://github.com/aws-amplify/amplify-swift/actions/runs/19873963503