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/lib/remote_census_api_spec.rb

@@ -26,7 +26,7 @@ describe RemoteCensusApi do
     end
 
     describe "request messages" do
-      let(:valid_response) { File.read("spec/fixtures/files/remote_census_api/valid.xml") }
+      let(:valid_response) { File.read(file_fixture("remote_census_api/valid.xml")) }
 
       def request_with(params)
         { "request" => params }