[devtools] add http_publish to CubicWebTC (closes #2565882)
Fakes a http request, without overriding the normal error handling.
Returns the request object so the caller can check for errors.
[testlib] url_publish should give url to the request and the rset returned by the path evaluator to ctrl_publish. Closes #2557468
Also, url_publish now accepts a `data` argument to simulate POST of values.