mock.swift

class MockUserDefault: UserDefaults {
    var balance = Double()
    override func set(_ value: Double, forKey defaultName: String) {
        balance = value
    }
}

func testUserBalanceCalculation() {
    let user = User(balance: 100)
    let mockUserDefaults = MockUserDefault()
    saveUserBalance(user: user, buyingAmount: 50, defaults: mockUserDefaults)
    XCTAssertTrue(mockUserDefaults.balance == 50)
}