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/Makefile b/src/Makefile
index bda65ab..0b95c6e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2242,6 +2242,7 @@
 	test_reltime \
 	test_retab \
 	test_ruby \
+	test_scroll_opt \
 	test_scrollbind \
 	test_search \
 	test_searchpos \