Add checkbox for linking spec text to test suite

Adding a script and a file of JSON data to link spec text to the relevant tests in the test suite.

Note that there is no point in editing the JSON data file - the source of this data is the META rel=help tags in each test file. When annotations are wrong (links end up in the wrong place or do not appear at all), the meta data within the tests need fixing.

(Stupidly I forgot to make a branch in my xhr fork for this :-/ but I hope you'll take it in so I'll only worry about that if you don't ;))