private func sharedFolderURL() -> URL {

    if let simulatorSharedDir = ProcessInfo().environment["SIMULATOR_SHARED_RESOURCES_DIRECTORY"] {
        // running on the simulator. We'll write to ~/Library/Caches
        let simulatorHomeDirURL = URL(fileURLWithPath: simulatorSharedDir)
        let cachesDirURL = simulatorHomeDirURL.appendingPathComponent("Library/Caches")
        XCTAssertTrue(FileManager.default.isWritableFile(atPath: cachesDirURL.path), "Cannot write to simulator Caches directory")
        let sharedFolderURL = cachesDirURL.appendingPathComponent("Secrets")
        XCTAssertNoThrow( try FileManager.default.createDirectory(at: sharedFolderURL, withIntermediateDirectories: true, attributes: nil), "Failed to create shared folder \(sharedFolderURL.lastPathComponent) in simulator Caches directory at \(cachesDirURL)")
        return sharedFolderURL
    }
    else {
        // running on the device. We'll write to the AppGroup folder
        guard let appGroupURL = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: AppGroupName) else {
            XCTFail("Failed to get URL for app group \(AppGroupName). Check your entitlements.")
            fatalError()
        }
        return appGroupURL
    }
}