Allow remote translation tests to be run offline

The method `available_locales` in
`RemoteTranslations::Microsoft::AvailableLocales` needs to execute a
request to an external server in order to work, meaning it will fail if
the machine its executed on doesn't have an internet connection.

So we're stubbing the method in the tests using it.

spec/controllers/remote_translation_controller_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-describe RemoteTranslationsController do
+describe RemoteTranslationsController, :remote_translations do
   describe "POST create", :delay_jobs do
     let(:debate) { create(:debate) }
 

spec/lib/remote_translations/caller_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-describe RemoteTranslations::Caller do
+describe RemoteTranslations::Caller, :remote_translations do
   before do
     RemoteTranslation.skip_callback(:create, :after, :enqueue_remote_translation)
   end

spec/models/remote_translation_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-describe RemoteTranslation do
+describe RemoteTranslation, :remote_translations do
   let(:remote_translation) { build(:remote_translation, locale: :es) }
 
   it "is valid" do

spec/spec_helper.rb

@@ -69,6 +69,11 @@ RSpec.configure do |config|
     Delayed::Worker.delay_jobs = false
   end
 
+  config.before(:each, :remote_translations) do
+    allow(RemoteTranslations::Microsoft::AvailableLocales)
+      .to receive(:available_locales).and_return(I18n.available_locales.map(&:to_s))
+  end
+
   config.before(:each, :with_frozen_time) do
     travel_to Time.current # TODO: use `freeze_time` after migrating to Rails 5.2.
   end