Skip tests that use a site which is failing

Add 'hostname' to TestCase.sites dict, and skip class when
failed to receive a useful response for the hosts homepage.

Re-enable weblib tests on travis-ci, instead using the new
automatic skip functionality to avoid errors due to WebCite
serving invalid XML and having an undecodable homepage.

Mention hostnames in test method docstring where the hostname
is not easily guessed.

Bug: T58963
Change-Id: I8fdcdaa0fab3b680d35b81b20a12ff5b786f779d
Merge "Skip tests that use a site which is failing"