patch 8.2.2031: some tests fail when run under valgrind

Problem:    Some tests fail when run under valgrind.
Solution:   Avoid timing problems.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 6fa7e3b..90d828b 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -2181,18 +2181,21 @@
   else
     let cmd = 'grep foo'
   endif
+
   let g:job = job_start(cmd, {})
+  sleep 50m  " give the job time to start
   call job_stop(g:job)
-  sleep 50m
-  call assert_equal(-1, job_info(g:job).exitval)
+  call WaitForAssert({-> assert_equal(-1, job_info(g:job).exitval)})
+
   let g:job = job_start(cmd, {})
+  sleep 50m
   call job_stop(g:job, 'term')
-  sleep 50m
-  call assert_equal(-1, job_info(g:job).exitval)
+  call WaitForAssert({-> assert_equal(-1, job_info(g:job).exitval)})
+
   let g:job = job_start(cmd, {})
-  call job_stop(g:job, 'kill')
   sleep 50m
-  call assert_equal(-1, job_info(g:job).exitval)
+  call job_stop(g:job, 'kill')
+  call WaitForAssert({-> assert_equal(-1, job_info(g:job).exitval)})
 endfunc
 
 func Test_issue_5485()