oauth_validator: Avoid races in log_check()

oauth_validator: Avoid races in log_check()

Commit e0f373ee4 fixed up races in Cluster::connect_fails when using
log_like. t/002_client.pl didn't get the memo, though, because it
doesn't use Test::Cluster to perform its custom hook tests. (This is
probably not an issue at the moment, since the log check is only done
after authentication success and not failure, but there's no reason to
wait for someone to hit it.)

Introduce the fix, based on debug2 logging, to its use of log_check() as
well, and move the logic into the test() helper so that any additions
don't need to continually duplicate it.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAOYmi%2BmrGg%2Bn_X2MOLgeWcj3v_M00gR8uz_D7mM8z%3DdX1JYVbg%40mail.gmail.com
Backpatch-through: 18