patch 8.0.0471: exit callback test sometimes fails

Problem:    Exit callback test sometimes fails.
Solution:   Add it to the list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 732cc0e..b0964a4 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -167,6 +167,7 @@
       \ 'Test_close_and_exit_cb()',
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',
+      \ 'Test_exit_callback_interval()',
       \ 'Test_nb_basic()',
       \ 'Test_oneshot()',
       \ 'Test_pipe_through_sort_all()',