Skip test if no ipv6 on system detected

[euri10]

Fixes #1281

[euri10]

ok with this fix, the ipv6 test is skipped as expected when disabling ipv6 with the following:

 sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
 sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1

however a side effect is that since /etc/hosts contains ::1 localhost ip6-localhost ip6-loopback then test called hostname fails, it cannot create the server on

uvicorn/uvicorn/server.py

Lines 148 to 156 in 632f33e

	server = await loop.create_server(
	create_protocol,
	host=config.host,
	port=config.port,
	ssl=config.ssl,
	backlog=config.backlog,
	)
	except OSError as exc:
	logger.error(exc)

because it resolves localhost to ::1

Care to test this @kloczek because I dont know if this will behave the same on a machine that has no ipv6 by default like the one you seem to have for running your test suite

[euri10]

checked in a fedora docker that has no ipv6 enabled and that looks fine, no ipv6 entries in /etc/hosts

[aminalaee]

Looks good