Use -Skip:$IsWindows

Uses the $IsWindows etc. booleans from the Microsoft.PowerShell.Platform
module instead of sourcing Test-Common repeatedly.

test/powershell/Add-Type.Tests.ps1

@@ -1,5 +1,5 @@
 Describe "Add-Type" {
-    It -Pending "Should not throw given a simple class definition" {
+    It "Should not throw given a simple class definition" -Skip:$IsWindows {
         { Add-Type -TypeDefinition "public static class foo { }" } | Should Not Throw
     }
 }