patch 8.2.0400: not all tests using a terminal are in the list of flaky tests

Problem:    Not all tests using a terminal are in the list of flaky tests.
Solution:   Introduce the test_is_flaky flag.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 7e3035a..84c46cf 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1868,6 +1868,8 @@
 func Test_Changed_FirstTime()
   CheckFeature terminal
   CheckNotGui
+  " Starting a terminal to run Vim is always considered flaky.
+  let test_is_flaky = 1
 
   " Prepare file for TextChanged event.
   call writefile([''], 'Xchanged.txt')