patch 8.2.5145: exit test causes spurious valgrind reports

Problem:    Exit test causes spurious valgrind reports.
Solution:   Skip test.  Add CheckNotValgrind.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index a7a609b..d299ee3 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1793,9 +1793,7 @@
 
 func Test_job_stop_immediately()
   " With valgrind this causes spurious leak reports
-  if RunningWithValgrind()
-    return
-  endif
+  CheckNotValgrind
 
   let g:job = job_start([s:python, '-c', 'import time;time.sleep(10)'])
   try