test_web.py: fix test_GET_DIRECTORY_html_filenode_encoding for Twisted==12.3.0. fixes #1922

twisted.web.html.escape was used to produce html-encoded string (to then look it up in "value" attribute), but behavior of that function has changed between Twisted 12.2.0 (simple custom implementation) and 12.3.0 (imported from stdlib cgi module).
cgi.escape() requires quote=True parameter to escape double-quote characters.

Fixed by simply using reliable cgi.escape to produce html-escaped string for tests.

Pretty sure I used twisted.web.html.escape because Nevow also used it, but looks like currently it properly quotes attribute values, so guess different func is used for attributes there.