iOS: Eliminate unguarded-availability opt-out

[cbracken]

Eliminates the opt-out of -Wunguarded-availability-new in the ios_test_flutter target.

Xcode 15 beta XCTest framework headers contained unguarded usage of iOS 17 API. This bug resulted in engine build failures.

This was fixed in a later XCTest framework release.

/Applications/Xcode_15-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers/XCUIAccessibilityAuditTypes.h:19:30: error: 'XCUIAccessibilityAuditType' is only available on iOS 17.0 or newer [-Werror,-Wunguarded-availability-new]
   19 | typedef NS_OPTIONS(uint64_t, XCUIAccessibilityAuditType) {
      |                              ^
/Applications/Xcode_15-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers/XCUIAccessibilityAuditTypes.h:19:30: note: 'XCUIAccessibilityAuditType' has been marked as being introduced in iOS 17.0 here, but the deployment target is iOS 13.0.0
/Applications/Xcode_15-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers/XCUIAccessibilityAuditTypes.h:19:9: note: annotate anonymous enum with an availability attribute to silence this warning
   19 | typedef NS_OPTIONS(uint64_t, XCUIAccessibilityAuditType) {
      |         ^
/Applications/Xcode_15-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator17.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:383:34: note: expanded from macro 'NS_OPTIONS'
  383 | #define NS_OPTIONS(_type, _name) CF_OPTIONS(_type, _name)
      |                                  ^
/Applications/Xcode_15-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator17.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFAvailability.h:155:96: note: expanded from macro 'CF_OPTIONS'
  155 | #define CF_OPTIONS(_type, _name) __attribute__((availability(swift,unavailable))) _type _name; enum __CF_OPTIONS_ATTRIBUTES : _name
      |

Issue: flutter/flutter#128958

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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