Move fixture_path out of support/rspec file which is used by fast_spec_helper, so that fast specs can be run on its own again.

Move `fixture_path` config together with use_transactional_fixtures and
use_instantiated_fixtures. These settings are all added by rspec-rails

https://github.com/rspec/rspec-rails/blob/8c6c9590b94916199950dc8a91a9741d3be30c7c/lib/rspec/rails/configuration.rb#L65-L67