patch 8.2.1369: MS-Windows: autocommand test sometimes fails

Problem:    MS-Windows: autocommand test sometimes fails.
Solution:   Do not rely on the cat command.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 70fbe01..e6c7ec0 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1618,7 +1618,7 @@
   write
   au! BufWritePre
 
-  if executable('cat')
+  if has('unix')
     write XtestFilter
     write >> XtestFilter