patch 8.2.4250: channel out callback test is flaky on Mac

Problem:    Channel out callback test is flaky on Mac.
Solution:   Assign high priority to the test process. (Ozaki Kiichi,
            closes #9653)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4250,
 /**/
     4249,
 /**/