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/test_timers.vim b/src/testdir/test_timers.vim
index 367e46f..caeb230 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -43,7 +43,7 @@
   let timer = timer_start(50, 'MyHandler', {'repeat': -1})
   sleep 200m
   call timer_stop(timer)
-  call assert_inrange(2, 4, g:val)
+  call assert_inrange(2, 5, g:val)
 endfunc
 
 func Test_with_partial_callback()