fix(tests): move MockNotificationCenter out of #if canImport(Testing) guard

TestData.swift references MockNotificationCenter unconditionally, so on
Xcode 15.4 where Swift Testing is unavailable the type was never compiled
and the release build failed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: glenn-klaviyo

Tests/KlaviyoSwiftTests/KlaviyoNotificationDelegateTests.swift

@@ -8,9 +8,6 @@
 @testable import KlaviyoSwift
 import UserNotifications
 
-#if canImport(Testing)
-import Testing
-
 // MARK: - Test Doubles
 
 /// Minimal `UNUserNotificationCenterDelegate` conformer used for object-identity assertions.
@@ -41,6 +38,9 @@ final class MockNotificationCenter: UserNotificationCenterProtocol {
 
 // MARK: - Tests
 
+#if canImport(Testing)
+import Testing
+
 // Note: the `klaviyo_automatic_push_tracking` plist key itself is verified manually
 // in the example app — `Bundle.main` in the test runner never carries it, and making
 // `Bundle` injectable would add abstraction beyond the current ticket's scope.