[pigeon] Consolidate platform test harnesses part 5 - macOS

[stuartmorgan-g]

Part of #2803

Completely removes the Cocoapod-based macOS test, and replaces it with a test in the shared harness that uses the same structure as all of the other native tests. This is still (like the removed test) only a sanity test that things compile on macOS, not a full re-test of the Swift logic that is already covered in the Swift iOS unit tests. (Once there are integration tests, we can run those on both platforms to get complete e2e testing on both platforms.)

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[gaaclarke]

Does the order on reviewing this matter, should we be waiting for #2803?

[stuartmorgan-g]

Does the order on reviewing this matter

In theory the order of 2-5 doesn't matter. I just needed to break them apart for troubleshooting.

should we be waiting for #2803?

The sequence of PRs replaces #2803; I'll close it once everything land and I'm sure I didn't miss any bits while breaking it apart.