I believe all these tests require the network, at least they seem to
fail if it's I run them without connecting my computer to the web.
I'm looking at this as part of packaging linkchecker for GNU Guix,
where the package is build and the tests are run in a isolated
environment, intentionally without network access, to avoid issues
with non-reproducible package builds.
Without this patch, tests would fail on IPv6 hosts with this
mysterious error:
```
_______________________________________________________________________ TestHttpMisc.test_html ________________________________________________________________________
tests/checker/test_http_misc.py:30: in test_html
self.obfuscate_test()
tests/checker/test_http_misc.py:51: in obfuscate_test
url = u"http://%s/" % iputil.obfuscate_ip(ip)
linkcheck/network/iputil.py:290: in obfuscate_ip
raise ValueError('Invalid IP value %r' % ip)
E ValueError: Invalid IP value '2a02:2e0:3fe:1001:7777:772e:2:85'
```
As it turns out, the test host (`www.heise.de`) does have an IPv6
record and our tests pass on Travis only because they do not have a
working IPv6 stack. I happen to have IPv6 at home and tests are broken
here, so add a quick workaround so tests pass again.
Ideally, we would not have to deal with this hack and would handle
"obfuscation" correctly, but I have yet to figure out what that test
actually does before fixing it properly.
