patch 8.1.0390: scrollbars are not tested

Problem:    Scrollbars are not tested.
Solution:   Add test_scrollbar() and a test.
diff --git a/src/version.c b/src/version.c
index a5f2512..f9e9cf4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    390,
+/**/
     389,
 /**/
     388,