REGTESTS: fix a reload race in abns_socket.vtc

This test issues a reload over the master CLI, but it is totally
possible that the master has not yet finished starting up the master
CLI when the command is issued, resulting in a failure. This was much
more visible on the new master-worker model, but definitely affects the
old one and could be the reason for this test to occasionally fail on
the CI.

reg-tests/seamless-reload/abns_socket.vtc

@@ -45,6 +45,7 @@ haproxy h1 -W -S -conf {
 } -start
 
 shell {
+    sleep 0.1  ;# let the master process fully start and bind
     echo "reload" | socat -t1000 TCP:${h1_mcli_addr}:${h1_mcli_port} - | grep 'Success=1'
 }