[Integration Test] Implement test sandbox API

[sacOO7]

• Implement test sandbox API as a part of test-suite.
• Currently, only unit tests are available, to test complex scenarios, we will need ably sandbox in place.

┆Issue is synchronized with this Jira Task by Unito

[sacOO7]

Based on the last discussion, the scope of this work has been significantly reduced and we will only focus on testing basic functionality of public API's using ably sandbox.

[sacOO7]

sandbox setup and tearDown involves minimal work and can be referred from https://github.com/ably-forks/laravel-echo/blob/master/tests/ably/setup/sandbox.ts

[sacOO7]

Although chat-UTS covers complex testing scenarios, we must set up lang. specific adapter for it.
Also, we can't test the new public API immediately without exposing relevant methods and also depends on relevant API test availability in the chat-UTS.
This task will only focus on setting up sandbox API that will be useful to test public API's without considering complex scenarios.

[sacOO7]

Need to create separate JIRA issue for kotlin-UTS-adapter

[lawrence-forooghian]

I’ve written some basic integration tests for the Swift SDK in ably-labs/ably-chat-swift#100