Moving FileSet definition to specs

Before this commit, the file_set.rb was something that all applications
would load into their production code.  Ideally, we don't want to do
that for the primary reason that our file_set directly inherits from
`ActiveFedora::Base`.  If a downstream FileSet were to not inherit from
`ActiveFedora:Base` we would have a mismatch error.

With this commit, we're saying that the IIIF print gem will require a
FileSet that conforms to the PCDM expectations.  But we don't dictate
it's implementation details in the downstream application.  We instead
dictate those implementation details in the test application.

Related to:

- #101

spec/spec_helper.rb

@@ -36,6 +36,7 @@
 require 'rspec/active_model/mocks'
 require 'selenium-webdriver'
 require 'webdrivers'
+require 'support/models/file_set.rb'
 
 # @note In January 2018, TravisCI disabled Chrome sandboxing in its Linux
 #       container build environments to mitigate Meltdown/Spectre

spec/support/models/file_set.rb

@@ -0,0 +1,7 @@
+##
+# iiif_print requires a file set model that is compatible with Hyrax assumptions.  We do not want to
+# add this to app/models because those are loaded in the downstream application; which can create
+# unexpected surprises.
+class FileSet < ActiveFedora::Base
+  include ::Hyrax::FileSetBehavior
+end