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

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.

(cherry picked from commit 1713eec36d347e7e7d6208980f0abd2db8d1cd81)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 2b0ba5fe349f80013fb028f523c3ad945a8c5ffa)
Signed-off-by: Willy Tarreau <w@1wt.eu>