[ADD] qweb: handling of t-att=mapping

Changed render_att_att to return an iterable of pairs instead of a pair, and
dispatched t-att on whether its result is a Mapping.

Also changed qweb test runner so it uses ordereddict for JSON mapping in
params, otherwise iteration order (and thus order of attributes in output) is
unpredictable and results don't/can't match expectations (as both are
strings).

Note that this relies on JS implementation details wrt iteration order of
mappings. Tests would probably be somewhat less brittle if rendering output
was parsed to XML... if that's possible (?)