patch 8.2.4802: test is not cleaned up

Problem:    Test is not cleaned up.
Solution:   Make test clean up after itself.  Avoid NUL. (closes #10233)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index fd36317..c4ba10f 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
Binary files differ