Make sure plugin generator works with dummy app

The dummy app is using sprockets, so we need to include sprocket-rails
if the sprockets is not skipped.

Fixes #43920.

railties/lib/rails/generators/app_base.rb

@@ -170,15 +170,13 @@ def web_server_gemfile_entry # :doc:
       end
 
       def asset_pipeline_gemfile_entry
-        return [] if options[:skip_asset_pipeline]
+        return if options[:skip_asset_pipeline]
 
         if options[:asset_pipeline] == "sprockets"
           GemfileEntry.floats "sprockets-rails",
             "The original asset pipeline for Rails [https://github.com/rails/sprockets-rails]"
         elsif options[:asset_pipeline] == "propshaft"
           GemfileEntry.floats "propshaft", "The modern asset pipeline for Rails [https://github.com/rails/propshaft]"
-        else
-          []
         end
       end
 

railties/lib/rails/generators/rails/plugin/templates/Gemfile.tt

@@ -14,6 +14,10 @@ group :development do
 end
 <% end -%>
 
+<% if engine? && !skip_sprockets? -%>
+<%= asset_pipeline_gemfile_entry %>
+
+<% end -%>
 <% if rails_prerelease? -%>
 # Your gem is dependent on a prerelease version of Rails. Once you can lock this
 # dependency down to a specific version, move it to your gemspec.

railties/test/generators/plugin_generator_test.rb

@@ -291,6 +291,39 @@ def test_ensure_that_migration_tasks_work_with_mountable_option
     assert $?.success?, "Command failed: #{output}"
   end
 
+  def test_ensure_that_sprokets_is_required_when_mountable
+    run_generator [destination_root, "--mountable"]
+    assert_file "Gemfile", /^gem "sprockets-rails"/
+  end
+
+  def test_ensure_that_sprokets_is_required_when_full
+    run_generator [destination_root, "--full"]
+    assert_file "Gemfile", /^gem "sprockets-rails"/
+  end
+
+  def test_ensure_that_sprokets_is_not_required_when_not_mountable_or_full
+    run_generator
+    assert_file "Gemfile" do |content|
+      assert_no_match(/sprockets-rails/, content)
+    end
+  end
+
+  def test_ensure_that_sprokets_is_not_required_when_assets_pipeline_is_skipped
+    run_generator [destination_root, "--skip-asset-pipeline", "--mountable"]
+
+    assert_file "Gemfile" do |contents|
+      assert_no_match(/sprockets-rails/, contents)
+    end
+  end
+
+  def test_ensure_that_sprokets_is_not_required_when_assets_pipeline_is_not_sprockets
+    run_generator [destination_root, "--asset-pipeline=propshaft", "--mountable"]
+
+    assert_file "Gemfile" do |contents|
+      assert_no_match(/sprockets-rails/, contents)
+    end
+  end
+
   def test_creating_engine_in_full_mode
     run_generator [destination_root, "--full"]
     assert_file "app/assets/stylesheets/bukkits"