functests: Disable system(-s -) test on Windows

Assume something with system() if second test hangs as well. Assume something 
with reading stdin if not.

test/functional/core/main_spec.lua

@@ -36,6 +36,7 @@ describe('Command-line option', function()
       os.remove(dollar_fname)
     end)
     it('treats - as stdin', function()
+      if helpers.pending_win32(pending) then return end
       eq(nil, lfs.attributes(fname))
       funcs.system(
         {nvim_prog_abs(), '-u', 'NONE', '-i', 'NONE', '--headless',