patch 8.0.1370: channel test for callback is flaky

Problem:    Channel test for callback is flaky.
Solution:   Add the test to thelist of flaky tests.

src/testdir/runtest.vim

@@ -251,6 +251,7 @@ let s:flaky = [
       \ 'Test_exit_callback_interval()',
       \ 'Test_nb_basic()',
       \ 'Test_oneshot()',
+      \ 'Test_out_cb()',
       \ 'Test_paused()',
       \ 'Test_pipe_through_sort_all()',
       \ 'Test_pipe_through_sort_some()',

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1370,
 /**/
     1369,
 /**/