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).