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).
Merge branch 'master' of github.com:ClashTheBunny/tahoe-lafs into ipv6