patch 8.2.0402: setting local instead of global flag

Problem:    Setting local instead of global flag.
Solution:   Prepend "g:" to "test_is_flaky".
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 84c46cf..e5dec89 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1869,7 +1869,7 @@
   CheckFeature terminal
   CheckNotGui
   " Starting a terminal to run Vim is always considered flaky.
-  let test_is_flaky = 1
+  let g:test_is_flaky = 1
 
   " Prepare file for TextChanged event.
   call writefile([''], 'Xchanged.txt')