patch 8.1.2106: no tests for dragging the mouse beyond the window

Problem:    No tests for dragging the mouse beyond the window.
Solution:   Add a test. (Dominique Pelle, closes #5004)
diff --git a/src/version.c b/src/version.c
index 229312b..cfd70c8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2106,
+/**/
     2105,
 /**/
     2104,