TST/BUG: Rename html encoding test files.

html test_encode fails on OSX 10.9.4 due to missing dash in `utf32` encoding
string.

The test encoding string is derived from a split of the test file name.

Only utf32 is affected but this commit changes the utf8 and utf16 containing filenames as well for the sake of consistency.

```======================================================================
ERROR: test_encode (pandas.io.tests.test_html.TestReadHtmlEncodingLxml)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jmorris/Code/pandas/pandas/io/tests/test_html.py", line 624, in test_encode
    from_string = self.read_string(f, encoding).pop()
  File "/Users/jmorris/Code/pandas/pandas/io/tests/test_html.py", line 619, in read_string
    return self.read_html(fobj.read(), encoding=encoding, index_col=0)
  File "/Users/jmorris/Code/pandas/pandas/io/tests/test_html.py", line 607, in read_html
    return read_html(*args, **kwargs)
  File "/Users/jmorris/Code/pandas/pandas/io/html.py", line 851, in read_html
    parse_dates, tupleize_cols, thousands, attrs, encoding)
  File "/Users/jmorris/Code/pandas/pandas/io/html.py", line 714, in _parse
    raise_with_traceback(retained)
  File "/Users/jmorris/Code/pandas/pandas/io/html.py", line 708, in _parse
    tables = p.parse_tables()
  File "/Users/jmorris/Code/pandas/pandas/io/html.py", line 178, in parse_tables
    tables = self._parse_tables(self._build_doc(), self.match, self.attrs)
  File "/Users/jmorris/Code/pandas/pandas/io/html.py", line 527, in _build_doc
    parser = HTMLParser(recover=False, encoding=self.encoding)
  File "/Users/jmorris/anaconda/envs/py27/lib/python2.7/site-packages/lxml/html/__init__.py", line 1664, in __init__
    super(HTMLParser, self).__init__(**kwargs)
  File "parser.pxi", line 1598, in lxml.etree.HTMLParser.__init__ (src/lxml/lxml.etree.c:100669)
  File "parser.pxi", line 792, in lxml.etree._BaseParser.__init__ (src/lxml/lxml.etree.c:93393)
LookupError: unknown encoding: 'utf32'
```

Test failure was not resolved by building the most recent lxml with static dependencies and most recent versions of libxml2 and libxslt.

I contacted the lxml mailing list. 

http://mailman-mail5.webfaction.com/pipermail/lxml/2014-July/007239.html

It was suggested that problem may be in OSX libiconv though `iconv -l`doesn't show the bug on my system.