[FIAM unit tests] Fix unit test failures running on Xcode 15 on macos-14

[paulb777]

Description

Three unit test failures occur. Fix and update the tests job in inappmessaging.yml to use macos-14

Reproducing the issue

From https://github.com/firebase/firebase-ios-sdk/actions/runs/8650156018/job/23718047345

Test Suite 'InAppMessagingDisplay-UITests.xctest' failed at 2024-04-11 20:36:56.005.
Executed 32 tests, with 3 failures (0 unexpected) in 3871.893 (3871.928) seconds
Test Suite 'All tests' failed at 2024-04-11 20:36:56.006.
Executed 32 tests, with 3 failures (0 unexpected) in 3871.893 (3871.930) seconds

Firebase SDK Version

10.24.0

Xcode Version

15.3

Installation Method

CocoaPods

Firebase Product(s)

In-App Messaging

Targeted Platforms

iOS

Relevant Log Output

No response

If using Swift Package Manager, the project's Package.resolved

Expand Package.resolved snippet

Replace this line with the contents of your Package.resolved.

If using CocoaPods, the project's Podfile.lock

Expand Podfile.lock snippet

Replace this line with the contents of your Podfile.lock!