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.
1 file changed