patch 8.1.0600: channel test is flaky

Problem:    Channel test is flaky.
Solution:   Add test to list of flaky tests.

src/testdir/runtest.vim

@@ -281,6 +281,7 @@ let s:flaky_tests = [
       \ 'Test_close_callback()',
       \ 'Test_close_handle()',
       \ 'Test_close_lambda()',
+      \ 'Test_close_output_buffer()',
       \ 'Test_close_partial()',
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    600,
 /**/
     599,
 /**/