Fix system tests

Rails 6.0 assumes the use of the `selenium-webdriver` gem, and so tries
to:

1. Require 'selenium/webdriver' - fix by adding an empty file with that
   path and adding it to $LOAD_PATH
2. Load `Selenium::WebDriver::*` constants due to the default `:chrome`
   for the `using:` argument - fix by adding a non-existent `:cuprite`
   browser so that we fall out of Rails' case statement (Cuprite ignores
   this value since it only works with Chrome anyway)

spec/rails_helper.rb

@@ -17,6 +17,11 @@ def has_action_cable_testing?
 
 require 'rspec/rails'
 
+file = Pathname('../tmp/workaround/selenium/webdriver.rb').expand_path(__dir__)
+file.dirname.mkpath unless file.dirname.directory?
+file.write('') unless file.exist?
+$LOAD_PATH << file.dirname.dirname.to_s
+
 Dir[Rails.root.join('spec/support/*.rb')].each { |file| require file }
 Dir[Rails.root.join('spec/support/shared_contexts/*.rb')].each { |file| require file }
 Dir[Rails.root.join('spec/support/shared_examples/*.rb')].each { |file| require file }

spec/support/capybara.rb

@@ -50,7 +50,8 @@
   end
 
   config.before(:each, type: :system, js: true) do
-    driven_by :cuprite
+    # `using: :chrome` (the default) errors because Rails tries to use selenium-webdriver
+    driven_by :cuprite, using: :cuprite
 
     @fp_debug_thread = Thread.new do
       Thread.current.abort_on_exception = true