Add infrastructure for current preference dependent tests

Author: jmschonfeld

Tests/FoundationInternationalizationTests/CurrentInternationalizationPreferencesActor.swift

@@ -0,0 +1,44 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if canImport(FoundationInternationalization)
+@testable import FoundationEssentials
+@testable import FoundationInternationalization
+#else
+@testable import Foundation
+#endif
+
+// This actor is private and not exposed to tests to prevent accidentally writing tests annotated with @CurrentInternationalizationPreferencesActor which may have suspension points
+// Using the global helper function below ensures that only synchronous work with no suspension points is queued
+@globalActor
+private actor CurrentInternationalizationPreferencesActor: GlobalActor {
+    static let shared = CurrentInternationalizationPreferencesActor()
+    
+    private init() {}
+    
+    @CurrentInternationalizationPreferencesActor
+    static func usingCurrentInternationalizationPreferences(
+        body: () throws -> Void // Must be synchronous to prevent suspension points within body which could introduce a change in the preferences
+    ) rethrows {
+        try body()
+        
+        // Reset everything after the test runs to ensure custom values don't persist
+        LocaleCache.cache.reset()
+        CalendarCache.cache.reset()
+        _ = TimeZoneCache.cache.reset()
+        _ = TimeZone.resetSystemTimeZone()
+    }
+}
+
+internal func usingCurrentInternationalizationPreferences(_ body: sending () throws -> Void) async rethrows {
+    try await CurrentInternationalizationPreferencesActor.usingCurrentInternationalizationPreferences(body: body)
+}