In this case (where data can't be copied from PROD to TEST), I would create a brand new site collection with its own content database in SharePoint 2013 PROD environment. Create new test libraries and lists and populate them with TEST items and documents, possibly similar contents as in the actual prod site collection. And then take full database backup of this content database and restore it in the TEST environment to upgrade to SharePoint 2016.
Here is a sample PowerShell script showing how to create a Site Collection with its own content database.
Add-PSSnapin Microsoft.SharePoint.PowerShell
$webAppUrl = "https://sharePoint2013.contoso.com"
$siteUrl = "https://sharePoint2013.contoso.com/sites/TestSiteColl"
$contentDbName = "WSS_Content_TESTSite"
$siteName = "Test 2013 Site Collection"
$contentOwnerAccount = "domain\OwnerAccount"
$contentDb = Get-SPContentDatabase -Identity $contentDbName -ErrorAction Continue
if($contentDb -eq $null)
{
$contentDb = New-SPContentDatabase -Name $contentDbName -WebApplication $webAppUrl -MaxSiteCount 1 -WarningSiteCount 0
}
$site = New-SPSite -Url $siteUrl -Name $siteName -ContentDatabase $contentDb -Template "OFFILE#1" -OwnerAlias $contentOwnerAccount