Add CI job for running macOS unit tests

[Widcket]

• [ ] All new/changed/fixed functionality is covered by tests (or N/A)
• [ ] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR adds a new GitHub actions job to run the native Swift tests on macOS. The existing steps for running the iOS unit tests were reused, and extracted into a composite action.

The existing setup-ios composite action was also renamed to setup-darwin.

The Xcode version used to run the tests was updated to 15.0.1.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (bd6557a) 96.36% compared to head (ae9e5a1) 96.30%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## macos-support/update-pod-dependencies #356 +/- ## ========================================================================= - Coverage 96.36% 96.30% -0.07% ========================================================================= Files 98 98 Lines 1596 1596 Branches 328 340 +12 ========================================================================= - Hits 1538 1537 -1 Misses 50 50 - Partials 8 9 +1 ``` | [Flag](https://app.codecov.io/gh/auth0/auth0-flutter/pull/356/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [auth0_flutter](https://app.codecov.io/gh/auth0/auth0-flutter/pull/356/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `100.00% <ø> (ø)` | | | [auth0_flutter_android](https://app.codecov.io/gh/auth0/auth0-flutter/pull/356/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `97.35% <ø> (ø)` | | | [auth0_flutter_ios](https://app.codecov.io/gh/auth0/auth0-flutter/pull/356/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `99.70% <ø> (-0.15%)` | :arrow_down: | | [auth0_flutter_platform_interface](https://app.codecov.io/gh/auth0/auth0-flutter/pull/356/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `87.03% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#carryforward-flags-in-the-pull-request-comment) to find out more.

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