tests: also allow "Protocol not supported" in test-http-proxy error
I've seen this in a (misconfigured) FreeBSD jail which has ::1 as an
entry for localhost, but IPv6 support is disabled in the jail. It took
me months to figure out what was going on (and I only figured it out
when tinyproxy.py got confused by similar IPv4-level misconfiguration
of the localhost domain in /etc/hosts.)
I don't feel strongly about this patch: on the one hand, it's papering
over a host-level misconfiguration, but on the other it avoids some
weird and hard to diagnose problems that can occur in weirdly
restricted environments.
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