tests: update for new error message on already defined property

Signed-off-by: Jean Privat <jean@pryen.org>
model: add `MMethod::is_toplevel`

Documentation will like to have such an information easily available.

Signed-off-by: Jean Privat <jean@pryen.org>
modelize_property: top_level methods can only redefine and be redefined by top_level methods

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add tests for top-level redefinitions

Signed-off-by: Jean Privat <jean@pryen.org>