REGTESTS: abortonclose: after retries, 503 is expected, not close

The abortonclose test was only expecting a close after all server
retries were exhausted, it didn't check for the pending 503, which
fails with new versions of vtest starting with commit 8d6c6bd
("Leak-plugging on barriers").

This may be backported, but carefully in case older versions would
really close without responding.

reg-tests/http-messaging/http_abortonclose.vtc

@@ -83,7 +83,8 @@ haproxy h1 -conf {
 # No server, wait all connection retries : SC--
 client  c1 -connect ${h1_fe1_sock} {
     txreq -url /c1
-    expect_close
+    rxresp
+    expect resp.status == 503
 } -run
 
 # Wait c1 log entry