Fix test failure on Windows [skip travis]

src/nvim/testdir/shared.vim

@@ -202,9 +202,17 @@ endfunc
 func GetVimProg()
   if empty($NVIM_TEST_ARG0)
     " Assume the script was sourced instead of running "make".
+    if has('win32')
+      return '..\\..\\..\\build\\bin\\nvim.exe'
+    else
       return '../../../build/bin/nvim'
     endif
+  endif
+  if has('win32')
+    return substitute($NVIM_TEST_ARG0, '/', '\\', 'g')
+  else
     return $NVIM_TEST_ARG0
+  endif
 endfunc
 
 " Get the command to run Vim, with -u NONE and --headless arguments.