Test specific behavior of javascript:install:webpack

This adds test coverage for specific behavior of the
`javascript:install:webpack` command.

test/webpack_installer_test.rb

@@ -5,6 +5,24 @@ class WebpackInstallerTest < ActiveSupport::TestCase
   include RailsAppHelpers
   include SharedInstallerTests
 
+  test "webpack installer" do
+    with_new_rails_app do
+      out, _err = run_installer
+
+      File.read("Procfile.dev").tap do |procfile|
+        assert_match "js: yarn build --watch", procfile
+      end
+
+      assert_match "STUBBED gem install foreman", out
+
+      assert File.exist?("webpack.config.js")
+
+      assert_match "STUBBED yarn add webpack", out
+      assert_match %r{STUBBED npm (?:set-script build |pkg set scripts.build=)webpack --config webpack.config.js}, out
+      assert_match "STUBBED yarn build", out
+    end
+  end
+
   private
     def run_installer
       stub_bins("gem", "yarn", "npm")