We have a mixin that adds extra data-related tests for resource items.
It'll do full equality checks of the object's extra data against the
extra data that the test sets. However, it's possible that an object
can contain extra data entries that the mixin doesn't know about, but is
correct for it to exist in the extra data. An example of this is a file
attachment's SHA256 checksum value, which gets put into the extra data
upon file attachment creation and whenever the item resource is
serialized.

This change allows test cases to set additional expected extra data for
the object, so that tests can account for this when checking that the
object's extra data is correct.

Ran unit tests.
Used in the upcoming file attachment SHA256 change where this

  fixed some unit tests.