patch 8.1.2404: channel test fails under valgrind

Problem:    Channel test fails under valgrind.
Solution:   Sleep a bit longer.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 5634b98..17cdb7c 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1860,7 +1860,8 @@
   " Terminate job by signal
   let cmd = ['sleep', '10']
   let job = job_start(cmd)
-  sleep 10m
+  " 10m usually works but 50m is needed when running Valgrind
+  sleep 50m
   call job_stop(job)
   call WaitForAssert({-> assert_equal("dead", job_status(job))})
   let info = job_info(job)