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/Make_all.mak b/src/Make_all.mak
index 8f0f4c1..f5f0552 100644
--- a/src/Make_all.mak
+++ b/src/Make_all.mak
@@ -168,6 +168,7 @@
 	test_stat \
 	test_statusline \
 	test_substitute \
+	test_suspend \
 	test_swap \
 	test_syn_attr \
 	test_syntax \