patch 8.1.2381: not all register related code is covered by tests

Problem:    Not all register related code is covered by tests.
Solution:   Add more test cases. (Yegappan Lakshmanan, closes #5301)
diff --git a/src/version.c b/src/version.c
index b513b69..62bf063 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2381,
+/**/
     2380,
 /**/
     2379,