patch 9.0.1387: scrollbar test sporadically fails

Problem:    Scrollbar test sporadically fails.
Solution:   Mark the scrollbar test as flaky. (Christian Brabandt,
            closes #12113)

src/testdir/test_gui.vim

@@ -718,8 +718,11 @@ func Test_set_guioptions()
 endfunc
 
 func Test_scrollbars()
-  new
+  " this test sometimes fails on CI
+  let g:test_is_flaky = 1
+
   " buffer with 200 lines
+  new
   call setline(1, repeat(['one', 'two'], 100))
   set guioptions+=rlb
 

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1387,
 /**/
     1386,
 /**/