REGTESTS: wait for proper return of enable server in cli add server test

Add an empty expect statement after the 'enable server' cli command.
This ensures that the command has been properly handled by haproxy and
its processing is over.

It should fix the unstable behavior of the test which causes reports of
503 even after the server has been enabled.

This should fix the github issue #1188.

reg-tests/server/cli_add_server.vtc

@@ -82,6 +82,7 @@ client c2 -connect ${h1_feS_sock} {
 
 haproxy h1 -cli {
 	send "enable server test/s1"
+	expect ~ ".*"
 }
 
 client c3 -connect ${h1_feS_sock} {