REGTEST: checks: Fix sync condition for agent-check

agent-check.vtc script fails time to time because the 2nd cli command is sent to
early. Waiting for the connection close in the s1 server should be enough to be
sure the server state is updated.
diff --git a/reg-tests/checks/agent-check.vtc b/reg-tests/checks/agent-check.vtc
index 99cc581..eb2e08e 100644
--- a/reg-tests/checks/agent-check.vtc
+++ b/reg-tests/checks/agent-check.vtc
@@ -9,6 +9,7 @@
   barrier b1 sync
   recv 5
   send "75%,max-conn:30,maint,down\n"
+  expect_close
   barrier b2 sync
 } -start