Migration of runtime tests to hypothesis based property testing

[abelcheung]

This issue tracks the progress of migrating existing runtime tests to hypothesis:

• test_elem_class_lookup.py
• test_iterparse.py
  • Depends on whether XML/HTMLPullParser options are tested or not (currently not)
• test_attrib.py (ec203cf)
• test_errorlog.py (f07ad28)
• test_etree_element.py
  • Magic methods and basic mixin methods (72d7380)
  • Content only elements (81eac99)
  • Attribute access methods (fed3bb8)
  • find* family methods. (c2c5bf8)
  • add* family methods (e02626e)
  • iter* family methods (4b078ca)
• html/test_beautifulsoup.py (cfe2af5)
• html/test_html5lib.py (e2c3ebd)
• html/test_link_func.py (bde5482)

Following files don't need converting:

• test_constants.py
• test_xinclude.py
• test_etree_element.py get*() family methods