Prevent DeprecationWarnings from being raised while running API tests.

* Explicitly pass `many=True` when instantiating a serializer with an iterable
* Add `request` to the context of subclasses of hyperlinked fields when directly testing the fields' classes.
* Instantiate serializers containing hyperlinked fields with contexts containing `request` when directly testing the serializers' classes.

One `DeprecationWarning` is still raised, but I'm not touching that with a 10-foot pole:

    apps/addons/models.py:360: DeprecationWarning: object.__new__() takes no parameters
      return super(Addon, cls).__new__(cls, *args, **kw)

Patches for that are welcome :wink:

r?