Improve tests for gettext messages

Like this:

raise Exception(gettext('msg_error_status_%s' % record.status))

But for the reused from different module, it should not be a problem as the other module should be a dependency.

I think we can have some improvement with testing.
We could have a generic test that check if ir.message id exists for ModelSQL._sql_constraint.
And we could have gettext raised error when running under test and log an error on normal run.