[unit tests] Add some infrastructure for OneSignalCore and OneSignalUser

[nan-li]

Description

One Line Summary

Rebuilding our unit tests by adding basic infrastructure to work with the modules OneSignalCore and OneSignalUser.

Details

• This PR is best reviewed commit by commit.
• Overall, to avoid undergoing the work of refactoring the whole SDK to really use dependency injection, we are doing a pseudo-form. The eventual vision is that everything can be injected.

1. (cleanup) Removed some unused things from the OneSignalClient

2. Make OneSignalCore class to set and access module's services

• Add a protocol for OneSignalClient, called IOneSignalClient (open to more ios-appropriate name for the protocol)
• In production, still use the original OneSignalClient.sharedClient but if unit tests set their own client, return that one

3. Now, access the client via OneSignalCore

• Change all OneSignalClient.sharedClient calls to OneSignalCore.sharedClient

4. Extract Operation Repo's poll interval into a constant

• Store in the variable POLL_INTERVAL_MS so that it can have a different value for testing vs non-testing

5. Add 2 mocking frameworks and 2 testing bundles

• Add mocking framework called OneSignalCoreMocks to simplify working with OneSignalCore in unit tests
  • Add helper method clearUserDefaults()
  • Create MockOneSignalClient class to replace OneSignalClient in unit tests
• Add mocking framework called OneSignalUserMocks to simplify working with OneSignalUser in unit tests
  • Add some helper methods to reset state between tests, WIP as we realize more state needs to be cleared
• Add unit testing bundle OneSignalCoreTests, meant to test OneSignalCore
  • Add one test testNotificationJson, to get started
• Add unit testing bundle OneSignalUserTests, meant to test OneSignalUser
  • Add one test testLoginSetsExternalId, to get started

6. (cleanup) Delete old "user model" test files

• Delete old "user model" test files UserModelObjcTests.m and UserModelSwiftTests.swift
  • These weren't real tests

7. Add separate swiftlint file for test modules to allow force casts

Motivation

Rebuilding unit tests

Scope

• Affects unit testing

Testing

Unit testing

• There are only 2 tests in the SDK
• To run them, go to the Test Navigator (diamond icon) and run the two testing bundles.

Manual testing

• Ran unit tests on emulator and physical iPhone with iOS 17.2
• Ran example app on physical iPhone with iOS 17.2

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes
• Unit Tests

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is 

[nan-li]

The CI succeeded at first because it was only testing the tests in UnitTests module (which includes 0 tests).

We are trying to make all testing bundles run, so we added UserTests and CoreTests to the UnitTestApp scheme.

However, after that change, now the CI is failing.
Still investigating.

[nan-li]

The CI now passes, and runs all the testing bundles, UnitTests, OneSignalCoreTests.xctest, and OneSignalUserTests.xctest.