ActionDispatch::TestProcess needs to access :file_fixture_path on Rails 6.1

I had a similar problem to @koenpunt in #2349 where `fixture_file_upload` stopped working.

```
     NoMethodError:
       undefined method `file_fixture_path' for RSpec::Rails::FixtureFileUploadSupport::RailsFixtureFileWrapper:Class
       Did you mean?  fixture_path
     # /actionpack/lib/action_dispatch/testing/test_process.rb:25:in `fixture_file_upload'
     # /rspec-rails-dd093928f021/lib/rspec/rails/fixture_file_upload_support.rb:5:in `fixture_file_upload'

```

In my case, it's because I'm on Rails 6.1 alpha. As of [these recent changes]( rails/rails#39086), `ActionController::TestCase`  now needs access to `fixture_file_path` within `fixture_file_upload`.

There's not currently any CI for Rails 6.1 (as it's not out yet) but I thought I would get a head start on things.

Author: sudara

lib/rspec/rails/fixture_file_upload_support.rb

@@ -23,6 +23,7 @@ class RailsFixtureFileWrapper
 
         class << self
           attr_reader :fixture_path
+          attr_reader :file_fixture_path
 
           # Get instance of wrapper
           def instance