[pigeon] Fully-qualify types in Equatable extension test (flutter#6946)

Updating to Xcode 16 exposed a new compilation error: "extension declares a conformance of imported type 'DataWithEnum' to imported protocol 'Equatable'".  The [Swift Warning for Retroactive Conformances of External Types](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0364-retroactive-conformance-warning.md) proposal says:

> It does mean projects building with an older Swift will not have access to [the @retroactive] syntax, so as a source compatible fallback, a client can silence this warning by fully-qualifying all types in the extension. As an example, the above conformance can also be written as
> ```
> extension Foundation.Date: Swift.Identifiable {
>     // ...
> }
> ```
> This will allow projects that need to build with multiple versions of Swift, and which have valid reason to declare such conformances, to declare them without tying their project to a newer compiler build.

So change the test to fully-qualify the types.  This prevented the warning when I tried it in flutter/packages#6942.

Fixes flutter#150391

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,7 @@
+## NEXT
+
+* [swift] Fully-qualifies types in Equatable extension test.
+
 ## 20.0.1
 
 * [cpp] Fixes handling of null class arguments.

packages/pigeon/platform_tests/test_plugin/example/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/EnumTests.swift

@@ -12,7 +12,7 @@ class MockEnumApi2Host: EnumApi2Host {
   }
 }
 
-extension DataWithEnum: Equatable {
+extension test_plugin.DataWithEnum: Swift.Equatable {
   public static func == (lhs: DataWithEnum, rhs: DataWithEnum) -> Bool {
     lhs.state == rhs.state
   }