Random Generator, Random Array Generator

Build some infrastructures, like random generator, random array generator, etc.

enum TestError : Error {
    case argumentFault
    case resultFault
}

extension Int {
    ///Generate a random int with an upper & lower bound
    static func random(above low:Int = 0, under high:Int = 9) throws -> Int {
        guard high >= low else { throw TestError.argumentFault }
        let range = high - low + 1;
        return Int(arc4random_uniform(UInt32(range)))+low;
    }
    
    ///Generate an integer array
    static func randomArray(above low:Int = 0, under high:Int = 9) throws -> [Int] {
        guard high >= low else { throw TestError.argumentFault }
        //Got a sequence
        var s = Array(low...high)
        
        //Randomize the order of elements. 
        //O(n) without taking the random algorithm into account
        for i in (0..