Tests for rollup

Ensures:

1. The rollup NPM packages are installed as dependencies
2. The rollup command is used as a build script
3. The Procfile.dev is copied / modified

test/installer/rollup_test.rb

@@ -0,0 +1,37 @@
+require "json"
+require "test_helper"
+
+class RollupInstallerTest < ActiveSupport::TestCase
+  include RailsAppHelpers
+
+  test "installer for rollup" do
+    with_new_rails_app do
+      _out, _err = run_command("bin/rails", "javascript:install:rollup")
+
+      assert_equal File.read("#{__dir__}/../../lib/install/Procfile.dev"), File.read("Procfile.dev")
+      assert_equal File.read("#{__dir__}/../../lib/install/rollup/rollup.config.js"), File.read("rollup.config.js")
+
+      JSON.parse(File.read("package.json")).tap do |package|
+        assert_not_nil package["dependencies"]["rollup"]
+        assert_not_nil package["dependencies"]["@rollup/plugin-node-resolve"]
+        assert_equal "rollup -c --bundleConfigAsCjs rollup.config.js", package["scripts"]["build"]
+      end
+
+      if sprockets?
+        assert_match "//= link_tree ../builds", File.read("app/assets/config/manifest.js")
+      end
+    end
+  end
+
+  test "installer with pre-existing Procfile" do
+    with_new_rails_app do
+      File.write("Procfile.dev", "pre: existing\n")
+      _out, _err = run_command("bin/rails", "javascript:install:rollup")
+
+      assert_equal <<~YAML, File.read("Procfile.dev")
+        pre: existing
+        js: yarn build --watch
+      YAML
+    end
+  end
+end