Match direct-child <br> and <em> in _stringify

[c-tonneslan]

_stringify turns <br> inside a table cell into a newline (and marks <em>), but the XPath *//br expands to child::*/descendant-or-self::node()/br — it only matches a <br> nested inside a child element of the cell. A <br> that is a direct child of the <td> is never matched, so the newline isn't inserted and two logical lines get glued together.

scraper._stringify(lxml.html.fromstring("<td>Line1<br/>Line2</td>"))
# before: 'Line1Line2'
# after:  'Line1\nLine2'

.//br is descendant-or-self::node()/br, a strict superset that also covers the nested case, so cells whose <br> sits inside a <font>/other wrapper (as in the current fixtures) behave exactly as before. Same fix for *//em → .//em.

Added a parametrized test in tests/test_parsing.py covering both the direct-child and nested cases (the direct-child case fails on the current XPath).