How to test a content type with a IContextAwareDefaultFactory

We have a Dexterity-based content type that must inherit a field's default value from its parent. We use the following:

In the model:

<model xmlns="http://ift.tt/VYKrdE"
       xmlns:indexer="http://ift.tt/1LEwIBR"
       xmlns:form="http://ift.tt/VYKrdG">
  <schema>
    ...
    <field name="subjects" type="zope.schema.Tuple" indexer:searchable="true">
      ...
      <defaultFactory>my.package.content.default_subjects</defaultFactory>
      ...
    </field>
  </schema>
</model>

The factory is declared like this:

from zope.schema.interfaces import IContextAwareDefaultFactory
...
@provider(IContextAwareDefaultFactory)
def default_subjects(context):
    return getattr(context, 'subjects', ())

This works fine when running an instance:

(Pdb) context
<MyType at /Plone/folder>
(Pdb) type(context)
<type 'Acquisition.ImplicitAcquisitionWrapper'>

But fails when running tests as context is not wrapped:

(Pdb) context
<MyType at test>
(Pdb) type(context)
<class 'my.package.content.MyType'>

How can I solve this?