fix: flaky FIRCLSSettingsTests.m

[ncooke3]

Formalizing a follow-up to #15601, now that it is more convincing from the nightlies that the dispatch_async is the cuprit to the FIRCLSSettingsTests flakes.

Using the same approach as in #15608. It's possible that the race condition is playing out within single test as opposed to across test cases. In that case, we'll need to drain the queue or add a waiter within each affected test case.

Update: Because the SDK dispatches to the global concurrent background queue, there is not a way to reliably "drain" that queue and it appears that the background queue's work may not finish in a reasonable time when running in the CI environment. So, a queue with a higher priority is injected into the class for more reliable testing.

I did three rounds of testing, checking for the FIRCLSSettingsTest flakes. None reported.

1. https://github.com/firebase/firebase-ios-sdk/actions/runs/20387547915
2. https://github.com/firebase/firebase-ios-sdk/actions/runs/20395779611
3. https://github.com/firebase/firebase-ios-sdk/actions/runs/20396945460

#no-changelog

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[ncooke3]

Using the same approach as in #15608. It's possible that the race condition is playing out within single test as opposed to across test cases. In that case, we'll need to drain the queue or add a waiter within each affected test case.

This appears to be the case based on CI results from 2bdb8d3

[paulb777]

This might be a still existing, but rarer flake - https://github.com/firebase/firebase-ios-sdk/actions/runs/20387547915/job/58591201190

[ncooke3]

This might be a still existing, but rarer flake - https://github.com/firebase/firebase-ios-sdk/actions/runs/20387547915/job/58591201190

@paulb777, I haven't looked into that one yet, but yes– it does appear to be rarer.