patch 8.1.1852: timers test is flaky

Problem:    Timers test is flaky.
Solution:   Accept a larger count.  Add test to list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 9b41b46..fe02d88 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -333,6 +333,7 @@
       \ 'Test_quotestar()',
       \ 'Test_raw_one_time_callback()',
       \ 'Test_reltime()',
+      \ 'Test_repeat_many()',
       \ 'Test_repeat_three()',
       \ 'Test_server_crash()',
       \ 'Test_terminal_ansicolors_default()',