REGTESTS: mark abns_socket as broken

REGTESTS: mark abns_socket as broken

This test is inherently racy. It regularly pops up on the CI, and I've
spent one hour chasing a bug that apparently doesn't exist, just because
I'm running it 10 times in a row and it reports from 4 to 8 failures
when built at -O2 and generally even more at -O0. The logs are very
confusing, often reporting that it failed with status 0, with nothing
else wrong. I suspect it might sometimes be the shell command that fails
if it executes faster than haproxy finishes to start up, which would
also explain the relation with the optimization level. E.g:

>  Testing with haproxy version: 2.2.0
>  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (3.006) exit=2
>  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (3.006) exit=2
>  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (3.009) exit=2
>  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (3.008) exit=2
>  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (3.007) exit=2
>  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (3.007) exit=2
>  6 tests failed, 0 tests skipped, 4 tests passed

Some of the failures include this, suggesting that some barriers could
help:
  ---- h1   haproxy h1 PID file check failed:
       Could not read PID file '/tmp/haregtests-2020-10-09_11-19-40.kgsDB4/vtc.30539.04dbea7f/h1/pid

Since it has been causing false positives and consumed way more
troubleshooting time than it saved, let's mark it as broken so that it
doesn't waste more time. We can bring it back when someone manages to
figure what the problem is.