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/version.c

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