patch 8.1.0600: channel test is flaky

Problem:    Channel test is flaky.
Solution:   Add test to list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 56bb3d3..89c1e3a 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -281,6 +281,7 @@
       \ 'Test_close_callback()',
       \ 'Test_close_handle()',
       \ 'Test_close_lambda()',
+      \ 'Test_close_output_buffer()',
       \ 'Test_close_partial()',
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',