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/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 2642958..f785679 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -144,6 +144,7 @@
 	    test_channel.res \
 	    test_charsearch.res \
 	    test_cindent.res \
+	    test_clientserver.res \
 	    test_cmdline.res \
 	    test_command_count.res \
 	    test_crypt.res \