Add generic unittest to check relation field definition
Following [1], we could probably have such test for ModelStorage instance fields.
[1] https://groups.google.com/d/msgid/tryton-dev/20160225092900.GS10143%40tetsuo
This instance will be upgraded to Heptapod 1.3.3 (GitLab 16.9.6) on 2023-04-25 at 18:00 UTC+2
Following [1], we could probably have such test for ModelStorage instance fields.
[1] https://groups.google.com/d/msgid/tryton-dev/20160225092900.GS10143%40tetsuo