[google_sign_in] Convert iOS tests to Swift (#10787)

Converts the existing native unit tests from Objective-C to Swift.

This was done as a few steps:
- Direct conversion of the existing code (first pass by Gemini, but I checked it over and did some fixup).
- Converted from XCTest to Swift Testing, for flutter/flutter#180787
- Replaced global state with new `loadGoogleServiceInfo` and `createTestPlugin` helpers, because there was no reason the test needed to be so stateful, and removing the state makes the test-specific setup easier to follow (e.g., a subset of the tests didn't actually use the test-global `plugin` instance, but instead replaced it with a local copy, which made things confusing when moving between tests). It also allows adding suites.
- Added suites for each method.
- Combined some tests into parameterized tests.

Part of flutter/flutter#119103
Part of flutter/flutter#180787

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in_ios/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 6.2.5
+
+* Updates native header visibility for Swift testing.
+
 ## 6.2.4
 
 * Updates to Pigeon 26.