DEV: Add a spec for assets:precompile:js_processor (#23220)

spec/tasks/assets_precompile_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+RSpec.describe "assets:precompile" do
+  before do
+    Rake::Task.clear
+    Discourse::Application.load_tasks
+  end
+
+  describe "assets:precompile:js_processor" do
+    it "compiles the js processor" do
+      out = capture_stdout { Rake::Task["assets:precompile:js_processor"].invoke }
+
+      expect(out).to match(%r{Compiled js-processor: tmp/js-processor})
+      path = out.match(/: (.+)/)[1]
+      expect(File.exist?(path)).to eq(true)
+    end
+  end
+end