Fix Windows support for Unix shells and powershell (#860)

Excerpt from `:h shell-powershell`:

  To use powershell (on Windows):

    set shell=powershell shellquote=( shellpipe=\| shellxquote=
    set shellcmdflag=-NoLogo\ -NoProfile\ -ExecutionPolicy\ RemoteSigned\ -Command
    set shellredir=\|\ Out-File\ -Encoding\ UTF8

test/test.vader

@@ -90,6 +90,7 @@ Execute (Print Interpreter Version):
 
 Include: workflow.vader
 Include: regressions.vader
+Include: functional.vader
 
 Execute (Cleanup):
   silent! call RmRf(g:temp_plugged)