REGTESTS: log: check that a ring's internal server may target a UNIX socket

REGTESTS: log: check that a ring's internal server may target a UNIX socket

This test covers the bug fixed by this commit:

    ce6071c3b BUG/MEDIUM: server: skip log backend addr checks for internal proxies

It declares a ring whose server targets a UNIX socket bound by a
log-forward section, itself relaying to a vtest syslog receiver. It
fails to parse on an affected, unpatched build ("log server address
family not supported for log backend server").

The test pins "chroot /" in its global section: when haproxy runs as
root without an explicit "chroot" directive, its automatic "chroot
auto" feature (641fe4f11) isolates the process into an empty directory
right after startup. That breaks the ring's own UNIX-socket "server"
connect, a runtime, repeatedly-retried client connection, unlike the
log-forward listener's "bind unix@..." which is already open before
the chroot happens. Pinning "chroot /" makes the test behave the same
whether run as root or not.

This must be backported wherever the fix above is backported.