Use the file_fixture helper in tests

This way we don't have to write `"spec/fixtures/files"` every time.

Note this method isn't included in factories. We could include it like
so:

```
FactoryBot::SyntaxRunner.class_eval do
  include ActiveSupport::Testing::FileFixtures
  self.file_fixture_path = RSpec.configuration.file_fixture_path
end
```

However, I'm not sure about the possible side effects, and since we only
use attachments in a few factories, there isn't much gain in applying
the monkey-patch.

spec/shared/models/document_validations.rb

@@ -22,7 +22,7 @@ shared_examples "document validations" do |documentable_factory|
   it "is valid for all accepted content types" do
     acceptedcontenttypes.each do |content_type|
       extension = content_type.split("/").last
-      document.attachment = File.new("spec/fixtures/files/empty.#{extension}")
+      document.attachment = File.new(file_fixture("empty.#{extension}"))
 
       expect(document).to be_valid
     end