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.
diff --git a/src/version.c b/src/version.c
index ef5ddae..6b0953f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    284,
+/**/
     283,
 /**/
     282,