Fix flaky `testGetCaptureFailedRequestsEnabled`

[philipphofmann]

GitHub action Run Link

https://github.com/getsentry/sentry-cocoa/actions/runs/8520084162/job/23335548804?pr=3813

Disabling PR

https://github.com/getsentry/sentry-cocoa/pull/3814

Description

Runs successfully locally. The SentryNetworkTracker is a singleton https://github.com/getsentry/sentry-cocoa/blob/b15521ec4775f623b296e1fca558ffac46447bb8/Sources/Sentry/SentryNetworkTracker.m#L51-L57

and frequently calls methods on the SentrySDK, such as https://github.com/getsentry/sentry-cocoa/blob/b15521ec4775f623b296e1fca558ffac46447bb8/Sources/Sentry/SentryNetworkTracker.m#L445

This means the SentryNetworkTracker has some global state that is bad for testability. We could remove some of the global state to fix this flaky test.