patch 8.1.2278: using "cd" with "exe" may fail

Problem:    Using "cd" with "exe" may fail.
Solution:   Use chdir() instead.
diff --git a/src/testdir/test_autochdir.vim b/src/testdir/test_autochdir.vim
index 98d1af7..1473854 100644
--- a/src/testdir/test_autochdir.vim
+++ b/src/testdir/test_autochdir.vim
@@ -21,6 +21,6 @@
   bwipe!
   au! DirChanged
   set noacd
-  exe 'cd ' . cwd
+  call chdir(cwd)
   call delete('samples/Xtest')
 endfunc