chore: fix stress tests

[thisisabhash]

Issue #

None

Description

This PR

• disables parallel testing in stress tests
• fixes testMultipleSubscriptions() in APIStressTests

Failing run: https://github.com/aws-amplify/amplify-swift/actions/runs/19480644472

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 66.91%. Comparing base (76c8ddc) to head (0ea38ba).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4108      +/-   ##
==========================================
+ Coverage   66.87%   66.91%   +0.03%     
==========================================
  Files        1129     1129              
  Lines       42585    42585              
==========================================
+ Hits        28478    28494      +16     
+ Misses      14107    14091      -16     

Flag	Coverage Δ
API_plugin_unit_test	68.38% <ø> (ø)
AWSPluginsCore	68.54% <ø> (ø)
Amplify	46.73% <ø> (+0.01%)	⬆️
Analytics_plugin_unit_test	84.84% <ø> (ø)
Auth_plugin_unit_test	72.46% <ø> (-0.09%)	⬇️
DataStore_plugin_unit_test	83.07% <ø> (+0.37%)	⬆️
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> (ø)
unit_tests	66.91% <ø> (+0.03%)	⬆️

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[thisisabhash]

Stress test successful run: https://github.com/aws-amplify/amplify-swift/actions/runs/19835803757