patch 8.1.1854: now another timer test is flaky

Problem:    Now another timer test is flaky.
Solution:   Add test to list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index fe02d88..7530bb9 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -336,6 +336,7 @@
       \ 'Test_repeat_many()',
       \ 'Test_repeat_three()',
       \ 'Test_server_crash()',
+      \ 'Test_stop_all_in_callback()',
       \ 'Test_terminal_ansicolors_default()',
       \ 'Test_terminal_ansicolors_func()',
       \ 'Test_terminal_ansicolors_global()',