[in_app_purchase_storekit] Address flaky tests

[LouiseHsu]

Fixes flutter/flutter#182845
Probably fixes flutter/flutter#183318

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[gemini-code-assist]

Code Review

This pull request addresses flaky tests in InAppPurchaseStoreKit2PluginTests.swift by skipping certain tests on a specific OS version known to have a StoreKit bug, and by increasing the timeout for two other tests. My review focuses on improving the maintainability of these changes. I've suggested extracting duplicated test-skipping logic into a helper function and replacing a magic number for a timeout with a named constant. These changes will make the test suite cleaner and easier to manage.

[LouiseHsu]

https://developer.apple.com/forums/thread/808030
.setSimulatedError doesnt work on specifically 26.2, but apples availability annotation doesnt support donut hole versioning.