Fix issue #1609 - Get-PSSession -computername fails on nano

Root cause: Microsoft.WSMan.Management  and  Microsoft.PowerShell.Commands.Diagnostics  are not put in the default snapin list for win-ops, and thus when a runspace is created using RunspaceConfiguration, those 2 are not loaded as snapins by default, which is a behavior change compared to Nano PS.
Fix: Replace the guard 'PORTABLE' to be 'UNIX' so that they are in the default snapin list when targeting windows platform.

test/powershell/engine/BasicEngine.Tests.ps1

@@ -3,5 +3,14 @@ Describe 'Basic engine APIs' -Tags "CI" {
         It 'can create default instance' {
             [powershell]::Create() | Should Not Be $null
         }
+
+        It "can load the default snapin 'Microsoft.WSMan.Management'" -skip:(-not $IsWindows) {
+            $ps = [powershell]::Create()
+            $ps.AddScript("Get-Command -Name Test-WSMan") > $null
+            
+            $result = $ps.Invoke()
+            $result.Count | Should Be 1
+            $result[0].PSSnapIn.Name | Should Be "Microsoft.WSMan.Management"
+        }
     }
 }