patch 8.0.0284: collapse buffers test failed once

Problem:    The Test_collapse_buffers() test failed once, looks like it is
            flaky.
Solution:   Add it to the list of flaky tests.

src/testdir/runtest.vim

@@ -153,12 +153,13 @@ endif
 
 " Names of flaky tests.
 let s:flaky = [
-      \ 'Test_reltime()',
-      \ 'Test_nb_basic()',
-      \ 'Test_communicate()',
       \ 'Test_close_and_exit_cb()',
+      \ 'Test_collapse_buffers()',
+      \ 'Test_communicate()',
+      \ 'Test_nb_basic()',
       \ 'Test_pipe_through_sort_all()',
       \ 'Test_pipe_through_sort_some()'
+      \ 'Test_reltime()',
       \ ]
 
 " Locate Test_ functions and execute them.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    284,
 /**/
     283,
 /**/