vim-patch:8.0.0471: exit callback test sometimes fails

Problem:    Exit callback test sometimes fails.
Solution:   Add it to the list of flaky tests.

0529b3eb01

src/nvim/testdir/runtest.vim

@@ -197,6 +197,7 @@ endif
 
 " Names of flaky tests.
 let s:flaky = [
+      \ 'Test_exit_callback_interval()',
       \ 'Test_with_partial_callback()',
       \ 'Test_oneshot()',
       \ 'Test_lambda_with_timer()',