Clean / Consolidate pandas/tests/io/test_html.py
This is an **extremely** aggressive change to get all of the test cases unified between the LXML and BS4 parsers. On the plus side both parsers can share the same set of tests and functionality, but on the downside it gives lxml a little more power than it had previously, where it would quickly fall back to bs4 for malformed sites. 

Review / criticism appreciated