patch 8.2.4728: no test that v:event cannot be modified

Problem:    No test that v:event cannot be modified.
Solution:   Add a test. (closes #10139)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 896c563..9af79d8 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
Binary files differ