patch 8.0.1237: ":set scroll&" often gives an error

Problem:    ":set scroll&" often gives an error.
Solution:   Don't use a fixed default value, use half the window height. Add a
            test. (Ozaki Kiichi, closes #2104)
diff --git a/src/testdir/test_alot.vim b/src/testdir/test_alot.vim
index ad13303..b8a5650 100644
--- a/src/testdir/test_alot.vim
+++ b/src/testdir/test_alot.vim
@@ -41,6 +41,7 @@
 source test_put.vim
 source test_recover.vim
 source test_reltime.vim
+source test_scroll_opt.vim
 source test_searchpos.vim
 source test_set.vim
 source test_sort.vim