Fix Capybara screenshots in feature specs

@chrislo & I accidentally removed the screenshotting capability in #6669 at the
same time as we removed the redundant configuration for running feature specs
in a browser using the cuprite driver, because we were under the mistaken
impression that rack-test driver did not support screenshots.

As @murugapl & @misaka pointed out, capybara-screenshot does actually support
HTML screenshots when using rack-test. These screenshots are improved by
setting `Capybara.asset_host = "http://localhost:4000"` so that assets can be
loaded if you have the Rails app running locally.

This commit partially reverts this commit [1] to restore the screenshotting
behaviour.

[1]: 0a95de7

Author: floehopper

Gemfile

@@ -117,6 +117,7 @@ end
 
 group :test do
   gem "capybara"
+  gem "capybara-screenshot"
   gem "capybara_accessible_selectors",
       github: "citizensadvice/capybara_accessible_selectors"
   gem "climate_control"

Gemfile.lock

@@ -240,13 +240,18 @@ GEM
       rack-test (>= 0.6.3)
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
+    capybara-screenshot (1.0.27)
+      capybara (>= 1.0, < 4)
+      launchy
     caxlsx (4.4.2)
       htmlentities (~> 4.3, >= 4.3.4)
       marcel (~> 1.0)
       nokogiri (~> 1.10, >= 1.10.4)
       rubyzip (>= 2.4, < 4)
     cgi (0.5.1)
     charlock_holmes (0.7.9)
+    childprocess (5.1.0)
+      logger (~> 1.5)
     climate_control (1.2.0)
     coderay (1.1.3)
     concurrent-ruby (1.3.6)
@@ -435,6 +440,10 @@ GEM
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     language_server-protocol (3.17.0.5)
+    launchy (3.1.1)
+      addressable (~> 2.8)
+      childprocess (~> 5.0)
+      logger (~> 1.6)
     lint_roller (1.1.0)
     listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
@@ -935,6 +944,7 @@ DEPENDENCIES
   bootsnap
   brakeman
   capybara
+  capybara-screenshot
   capybara_accessible_selectors!
   caxlsx
   charlock_holmes

spec/spec_helper.rb

@@ -108,11 +108,13 @@
 end
 require "rspec/rails"
 # Add additional requires below this line. Rails is not loaded until this point!
+require 'capybara-screenshot/rspec'
 require "rack_session_access/capybara"
 require "console"
 
 Faker::Config.locale = "en-GB"
 
+Capybara.asset_host = "http://localhost:4000"
 Console.logger.off!
 Capybara.server = :falcon