REGTESTS: mark abns_socket as working now

REGTESTS: mark abns_socket as working now

William noticed that the real issue in the abns test was that it was
failing to rebind some listeners, issues which were addressed in the
previous commits (in some cases, the master would even accept traffic
on the worker's socket which was not properly disabled, causing some
of the strange error messages).

The other issue documented in commit 2ea15a080 was the lack of
reliability when the test was run in parallel. This is caused by the
abns socket which uses a hard-coded address, making all tests in
parallel to step onto each others' toes. Since vtest cannot provide
abns sockets, we're instead concatenating the number of the listening
port that vtest allocated for another frontend to the abns path, which
guarantees to make them unique in the system.

The test works fine in all cases now, even with 100 in parallel.