patch 8.1.1651: suspend test is flaky on some systems

Problem:    Suspend test is flaky on some systems.
Solution:   Wait for the shell prompt to show. (Yee Cheng Chin, closes #4632)
diff --git a/src/testdir/test_suspend.vim b/src/testdir/test_suspend.vim
index 8b326f5..4b3bd5e 100644
--- a/src/testdir/test_suspend.vim
+++ b/src/testdir/test_suspend.vim
@@ -54,6 +54,8 @@
   " Quit gracefully to dump coverage information.
   call term_sendkeys(buf, ":qall!\<CR>")
   call term_wait(buf)
+  " Wait until Vim actually exited and shell shows a prompt
+  call WaitForAssert({-> assert_match('[$#] $', term_getline(buf, '.'))})
   call StopShellInTerminal(buf)
 
   exe buf . 'bwipe!'