Like https://github.com/w3c/web-platform-tests/pull/8895 for quirks,
this adapts to https://github.com/whatwg/xhr/pull/184, after which the
wpt links are generated and match the shortname.
https://xhr.spec.whatwg.org/ currently has a 404 link to
https://github.com/w3c/web-platform-tests/tree/master/xhr.

Steps taken:
 * rename the directory
 * sed 's#/XMLHttpRequest#/xhr#g' everywhere
 * manually handle a few remaining cases of "XMLHttpRequest/"
 * look at ouput of grep -E '\SXMLHttpRequest' and 'XMLHttpRequest[^(;]'