Improve Engine's RSpec auto-integration

Nest test framework integration code within [initializer][] blocks. From
within those blocks, access the configuration through the
[Rails::Application#config][] attribute.

For `RSpec`, it's possible for the `RSpec` constant to exist prior to
the `RSpec.configure` module method. Drawing from prior art (like
[capybara's integration with RSpec][capybara/rspec], requiring
`rspec/core` appears to be a predictable way to guarantee the definition
of `RSpec.configure`. When `rspec/core` is already loaded, the `require`
statement will be a no-op.

[initializer]: https://api.rubyonrails.org/classes/Rails/Application.html#method-i-initializer
[Rails::Application#config]: https://edgeapi.rubyonrails.org/classes/Rails/Application.html
[capybara/rspec]: https://github.com/teamcapybara/capybara/blob/6267ff65a294a7dcbe9d7281c30cc2d91b08b093/lib/capybara/rspec.rb#L3

lib/capybara_accessibility_audit/engine.rb

@@ -10,27 +10,33 @@ class Engine < ::Rails::Engine
     ]
     config.capybara_accessibility_audit.audit_enabled = true
 
-    ActiveSupport.on_load :action_dispatch_system_test_case do
-      include CapybaraAccessibilityAudit::AuditSystemTestExtensions
+    initializer "capybara_accessibility_audit.minitest" do |app|
+      ActiveSupport.on_load :action_dispatch_system_test_case do
+        include CapybaraAccessibilityAudit::AuditSystemTestExtensions
 
-      self.accessibility_audit_enabled = Rails.configuration.capybara_accessibility_audit.audit_enabled
+        self.accessibility_audit_enabled = app.config.capybara_accessibility_audit.audit_enabled
 
-      accessibility_audit_after Rails.configuration.capybara_accessibility_audit.audit_after
+        accessibility_audit_after app.config.capybara_accessibility_audit.audit_after
+      end
     end
 
-    if defined?(RSpec)
-      RSpec.configure do |config|
-        config.include CapybaraAccessibilityAudit::AuditSystemTestExtensions, type: :system
-        config.include CapybaraAccessibilityAudit::AuditSystemTestExtensions, type: :feature
+    initializer "capybara_accessibility_audit.rspec" do |app|
+      if defined?(RSpec)
+        require "rspec/core"
 
-        configure = proc do
-          self.accessibility_audit_enabled = Rails.configuration.capybara_accessibility_audit.audit_enabled
+        RSpec.configure do |config|
+          config.include CapybaraAccessibilityAudit::AuditSystemTestExtensions, type: :system
+          config.include CapybaraAccessibilityAudit::AuditSystemTestExtensions, type: :feature
 
-          accessibility_audit_after Rails.configuration.capybara_accessibility_audit.audit_after
-        end
+          configure = proc do
+            self.accessibility_audit_enabled = app.config.capybara_accessibility_audit.audit_enabled
 
-        config.before(type: :system, &configure)
-        config.before(type: :feature, &configure)
+            accessibility_audit_after app.config.capybara_accessibility_audit.audit_after
+          end
+
+          config.before(type: :system, &configure)
+          config.before(type: :feature, &configure)
+        end
       end
     end
   end