patch 8.0.0477: the client-server test may hang when failing

Problem:    The client-server test may hang when failing.
Solution:   Set a timer.  Add assert_report()
diff --git a/src/if_xcmdsrv.c b/src/if_xcmdsrv.c
index 18c208b..9ff6d76 100644
--- a/src/if_xcmdsrv.c
+++ b/src/if_xcmdsrv.c
@@ -596,6 +596,10 @@
 	if (seconds >= 0 && (now - start) >= seconds)
 	    break;
 
+#ifdef FEAT_TIMERS
+	check_due_timer();
+#endif
+
 	/* Just look out for the answer without calling back into Vim */
 	if (localLoop)
 	{