TST: Mark test_html tests as network.

For right now, this prevents barfing with no connection. For the future
we should consider storing some or all of this data (esp given we don't
rely on them as data sources, they're just examples of what ought to
work). I have a separate branch (jtratner/pandas@dfb2a870a75e78188cd013f2553bf3b56e82bfc9) that stores the file that they are testing and has them use the file locally instead, if you want to use that instead, can make a PR>