Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

test/test_helper.rb

@@ -1,6 +1,17 @@
 ENV["RAILS_ENV"] ||= "test"
 require_relative "../config/environment"
 require "rails/test_help"
+require "minitest/mock"
+require "minitest/autorun"
+require "mocha/minitest"
+
+VCR.configure do |config|
+  config.cassette_library_dir = "test/vcr_cassettes"
+  config.hook_into :webmock
+  config.ignore_localhost = true
+  config.default_cassette_options = { erb: true }
+  config.filter_sensitive_data("<SYNTH_API_KEY>") { ENV["SYNTH_API_KEY"] }
+end
 
 module ActiveSupport
   class TestCase
@@ -16,3 +27,5 @@ module ActiveSupport
     end
   end
 end
+
+Dir[Rails.root.join("test", "interfaces", "**", "*.rb")].each { |f| require f }