Update parserTests from upstream; get linkprefix test; add :is: config.

Add new linkprefix test using iswiki.  Add is.json configuration to make
the test work.  The linkprefix test currently works for wt2html, but fails
for all other parser test modes (and fails on api/server.js as well).

Change-Id: I863553de700bbe4c0a35b739c44019ba02aa7ccc
Merge "Update parserTests from upstream; get linkprefix test; add :is: config."