XD7WaitForSite TEST and SET functions fail when used with psdscrunascredential

[matt6697]

The following code block make the TEST and SET functions of the XD7WaitForSite DSC resource fail when the resource is called using pssdcrunascredential instead of the credential parameter.

When $Crededential parameter is missing, the TestXDSite function throws an error.

$xdSiteName = TestXDSite -ExistingControllerName $ExistingControllerName -Credential $Credential;

The $Credential parameter is a mandatory parameter of the TestXDSite function (ValidateNotNull)

function TestXDSite {
<#
    .SYNOPSIS
        Checks whether the Citrix XenDesktop 7.x is available
#>
    [CmdletBinding()]
    [Diagnostics.CodeAnalysis.SuppressMessageAttribute('PSAvoidUsingEmptyCatchBlock', '')]
    param (
        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [System.String] $ExistingControllerName,

        [Parameter()]
        [ValidateNotNull()]
        [System.Management.Automation.PSCredential]
        [System.Management.Automation.CredentialAttribute()]
       $Credential

[matt6697]

@iainbrighton : Additional PR #14 needed to make it work with psdscrunascredential. Could you review and merge ? Cheers.