Extract Capybara setup to a helper file

Any Capybara-related test setup has been moved to `capybara_helper.rb`.
Now we just must require this file in order to run tests with Capybara.

I also moved the `#console_log` helper to this file to showcase how
other global helper methods could be defined in this file.

test/helper.rb

@@ -32,7 +32,6 @@ ENV['RACK_ENV'] = 'test'
 require 'gollum'
 require 'gollum/app'
 
-CAPYBARA_DRIVER = ENV['CI'] ? :selenium_chrome_headless : ENV.fetch('CAPYBARA_DRIVER', :selenium_chrome).to_sym
 
 # Disable the metadata feature
 $METADATA = false