patch 8.0.0267: channel test sometimes fails on Mac

Problem:    A channel test sometimes fails on Mac.
Solution:   Add the test to the list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 3ba3b02..c7028d2 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -156,6 +156,7 @@
       \ 'Test_reltime()',
       \ 'Test_nb_basic()',
       \ 'Test_communicate()',
+      \ 'Test_close_and_exit_cb()',
       \ 'Test_pipe_through_sort_all()',
       \ 'Test_pipe_through_sort_some()'
       \ ]