Enable previews by default in test environment.

Previews are currently enabled in development environment only.
Since previews can be very useful for testing,
enable them by default in test environment too.

lib/view_component/engine.rb

@@ -12,7 +12,7 @@ class Engine < Rails::Engine # :nodoc:
       options = app.config.view_component
 
       options.render_monkey_patch_enabled = true if options.render_monkey_patch_enabled.nil?
-      options.show_previews = Rails.env.development? if options.show_previews.nil?
+      options.show_previews = Rails.env.development? || Rails.env.test? if options.show_previews.nil?
       options.preview_route ||= ViewComponent::Base.preview_route
       options.preview_controller ||= ViewComponent::Base.preview_controller