patch 9.0.1387: scrollbar test sporadically fails

Problem:    Scrollbar test sporadically fails.
Solution:   Mark the scrollbar test as flaky. (Christian Brabandt,
            closes #12113)
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index 6b5e12c..809dd35 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -718,8 +718,11 @@
 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