oldtests: win: set shellpipe for cmd.exe

Redirect errors from :make to error file.
Fix execption error in test_compiler.vim on Windows.

src/nvim/testdir/setup.vim

@@ -33,4 +33,5 @@ if has('win32')
   let $TERM = ''
   let &shell = empty($COMSPEC) ? exepath('cmd.exe') : $COMSPEC
   set shellcmdflag=/s/c shellxquote=\" shellredir=>%s\ 2>&1
+  let &shellpipe = &shellredir
 endif