chore: various tweaks to support running tests in the CI

spec/rails_helper.rb

@@ -1,4 +1,4 @@
-ENV['POSTAL_CONFIG_ROOT'] = File.expand_path('../config', __FILE__)
+ENV['POSTAL_CONFIG_ROOT'] ||= File.expand_path('../config', __FILE__)
 
 require File.expand_path('../../config/environment', __FILE__)
 require 'rspec/rails'
@@ -6,6 +6,9 @@ require 'spec_helper'
 require 'factory_bot'
 require 'database_cleaner'
 
+DatabaseCleaner.allow_remote_database_url = true
+ActiveRecord::Base.logger = Logger.new("/dev/null")
+
 FACTORIES_EXCLUDED_FROM_LINT = []
 Dir[File.expand_path('../factories/*.rb', __FILE__)].each { |f| require f }