patch 8.1.0428: the :suspend command is not tested

Problem:    The :suspend command is not tested.
Solution:   Add a test. (Dominique Pelle, closes #3472)
diff --git a/src/testdir/test_alot.vim b/src/testdir/test_alot.vim
index ea829e0..3a0c6e0 100644
--- a/src/testdir/test_alot.vim
+++ b/src/testdir/test_alot.vim
@@ -52,6 +52,7 @@
 source test_sort.vim
 source test_sha256.vim
 source test_statusline.vim
+source test_suspend.vim
 source test_syn_attr.vim
 source test_tabline.vim
 source test_tabpage.vim