REGTESTS: mark proxy_protocol_random_fail as broken

REGTESTS: mark proxy_protocol_random_fail as broken

As indicated in issue #907 it very frequently fails on FreeBSD, and
looking at it, it's broken in multiple ways. It relies on log ordering
between two layers, the first one being allowed to support h2. Given
that on FreeBSD it usually ends up in timeouts, it's very likely that
for some reason one frontend logs before the other one or that curl
uses h2 instead of h1 there, and that the log instance waits forever
for its lines.

Usually it works fine when run locally though, so let's not remove it
and mark it as broken instead so that it can still be used when relevant.