patch 8.1.0276: no test for 'incsearch' highlighting with :s

Problem:    No test for 'incsearch' highlighting with :s.
Solution:   Add a screendump test.
diff --git a/src/version.c b/src/version.c
index a8262a7..98b2335 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    276,
+/**/
     275,
 /**/
     274,