Delete redundant (skipped) tests

Attachments are no longer created with mass asssignment on editions
so these tests are redundant. They were being skipped anyway...
Drop redundant attachment test macro

The AdminEditionAttachableControllerTestHelpers module used to provide
a macro for testing the mass-assignment of attachments on editions.
This is no longer required as edition attachments now have their own
separate interface and are no longer mass-assigned. The tests that
are still relevant for corporate information pages have been moved
inline.