Rewrite test_allow_arbitrary_field_names_in_error
Rewrite test_allow_arbitrary_field_names_in_error

The former implementation of this test illustrates a use case involving
a schema validator that is now useless since its function (raising an
error on unknown keys) is a feature of Marshmallow. This rewrite makes
the test simpler and more neutral.

marshmallow-code/marshmallow,mwstobo/marshmallow