[FIX] test_new_api,web: tests 9.0 adjustments

Rev e37ba618 forward-ported 8.0 revision f02b2300 from PR #12379.
This patch introduces extra tests and views for o2m onchange behavior,
including new fields in the test models.

In 9.0 a new convert_to_onchange() mechanism had been
introduced at 9f81c6d6

The combination of both requires some adaptation to the
expected results of the new tests and adaptation of existing tests.

The test models in `test_new_api` have an implicit depencies between
models which cannot be express on ORM:
    messages depends on participants of their discussion.

So, when editing a discussion, the field `participants` should be
processed before field `messages`. This is not always done because
field processing order depends of the iteration order of a `dict` (which
is undefined in python).
The testing tour `widget_x2many` worked by chance until now.
Adapt the tour to save the participants before the messages.