patch 8.1.0031: terminal test aucmd_on_close if flaky

Problem:    Terminal test aucmd_on_close if flaky.
Solution:   Wait a bit longer.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 41a0a45..3d06eb3 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -384,7 +384,8 @@
 func s:get_sleep_cmd()
   if s:python != ''
     let cmd = s:python . " test_short_sleep.py"
-    let waittime = 500
+    " 500 was not enough for Travis
+    let waittime = 900
   else
     echo 'This will take five seconds...'
     let waittime = 2000