patch 8.0.0474: the client-server feature is not tested

Problem:    The client-server feature is not tested.
Solution:   Add a test.
diff --git a/src/Makefile b/src/Makefile
index 1af1e17..315548b 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2097,11 +2097,12 @@
 	test_breakindent \
 	test_bufwintabinfo \
 	test_cdo \
+	test_changedtick \
 	test_channel \
 	test_charsearch \
 	test_charsearch_utf8 \
-	test_changedtick \
 	test_cindent \
+	test_clientserver \
 	test_cmdline \
 	test_command_count \
 	test_crypt \