How to make a singleton fully testable (resettable)?

SingleInstance.swift

import UIKit
import PlaygroundSupport

class SingleInstance {
    
    #if TESTING
    open static var shared = SingleInstance()
    static func reset() {
        SingleInstance.shared = SingleInstance()
    }
    #else
    public static let shared = SingleInstance()
    #endif

    private init() {}
    
    func greeting() {
        print("hello from \(String.pointer(self))")
    }
}

// But how to re-instantiate the SingleInstance?

extension String {
    static func pointer(_ object: AnyObject?) -> String {
        guard let object = object else { return "nil" }
        let opaque: UnsafeMutableRawPointer = Unmanaged.passUnretained(object).toOpaque()
        return String(describing: opaque)
    }
}

SingleInstance.shared.greeting()
SingleInstance.reset()
SingleInstance.shared.greeting()
print("done")