macOS Testing Strategy definition

[QuintinWillison]

I've created this issue to follow up on https://github.com/ably/ably-cocoa/pull/1543#discussion_r1034990139.

There is little benefit in me repeating the conversation had between @lawrence-forooghian and myself there, though I'll attempt a brief summary of where we probably need to go with this:

• Pin macOS GitHub runner versions used to build the SDK and the tests (stop using macos-latest)
• Document our strategy for proactively updating pinned macOS versions over time as new macOS and/or Xcode versions get released, including whether this can be (semi-)automated in some way
• Consider the macOS GitHub runner version (versions? - matrix testing?) that we run macOS tests under (the test target) - probably tied to the version that we have to use to build those tests too, because Apple

[sync-by-unito[bot]]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3115