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

### 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

```json

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

```

### If using CocoaPods, the project's Podfile.lock

Expand Podfile.lock snippet

```yml

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

```