chore: Add e2e test guide

apps/core/guides/features/end-to-end-testing.md

@@ -0,0 +1,106 @@
+# End to End Testing
+
+Legendary integrates [Cypress](https://cypress.io) for integration and end-to-end (E2E)
+testing. In our opinion, Cypress is the best toolkit available for testing end-to-end
+in-browser experiences. It has several features that make writing end-to-end tests as
+easy as possible:
+
+- **Rock-solid builds.** In our experience, selenium and chromedriver-based setups
+  break frequently on even minor version bumps. Every version of Cypress has worked
+  as intended for us.
+- **Automatic waiting.** Cypress handles async code without needing to manually
+  sprinkle waits and sleeps through your test code. I know your test framework
+  _says_ it does auto-waiting. Auto-waiting in Cypress actually works.
+- **Debugging.** You can use Chrome DevTools (or the equivalent in your favorite
+  browser) to debug your tests.
+- **Automatic videos of your test runs.**
+
+## A Word of Advice from Your Pal Robert
+
+Don't over-do it with integration and end-to-end testing. It's tempting to end-to-end
+test everything. After all, end-to-end tests _should_ catch any issue in your application,
+no matter what layer it happens in: model, view, controller, or even JavaScript.
+So what's the problem?
+
+For one, when compared to unit tests, integration and
+E2E tests are more labor intensive. It usually takes less time to write several
+unit tests covering all the aspects of your feature than it does to write one
+end-to-end scenario.
+
+Secondly, integration and e2e tests are good are identifying that you have a problem, but
+not where that problem occurs. They cast a broad net. When an e2e test fails, it
+does not point to a specific broken function or module. Unit tests do.
+
+Thirdly, E2E tests are slow. They have to wait for your page and
+all the related assets to load over a real HTTP connection. This is orders of magnitude
+slower than calling some functions.
+
+It's best to stick to the [Test Pyramid](https://martinfowler.com/articles/practical-test-pyramid.html).
+The majority of your tests should be unit tests and the majority of your code
+should be tested by unit tests.
+
+![Test Pyramid](assets/testing-pyramid.svg)
+
+## Running E2E Tests
+
+To run your Cypress tests, do:
+
+```
+npm run test:integration
+```
+
+## Writing E2E Tests
+
+[Read the Cypress guide to Writing Your First Test](https://docs.cypress.io/guides/getting-started/writing-your-first-test) to get started writing e2e tests with Cypress.
+
+
+### Preparing Your Database
+
+In most cases, you'll
+need to prepare your database first. Legendary gives you a tool for doing this
+called _seed sets_. Seed sets are Elixir scripts that your Cypress suite can
+invoke to make sure the database is in the needed state at the start of a test.
+
+Check the `apps/app/test/seed_sets` directory. Each seed set is just a .exs file
+which can be executed with a Cypress command. There's an example `apps/app/test/seed_sets/blog.exs`
+which prepares the database with a blog post. It looks like this:
+
+```elixir
+alias Legendary.Content.Post
+alias Legendary.Content.Repo
+
+%Post{
+  title: "Public post",
+  name: "public-post",
+  status: "publish",
+  type: "post",
+  date: ~N[2020-01-01T00:00:00],
+}
+|> Repo.insert!()
+```
+
+You can put whatever Elixir code you need in a seed_set!
+
+From your Cypress tests, you can run a seed_set like:
+
+```js
+cy.setupDB("app", "blog")
+```
+
+This invokes the seed set called blog (i.e. blog.exs) from the Elixir app called
+`app` (i.e. your app within the Legendary framework). See
+`apps/app/cypress/integration/blog_spec.js` for the complete example.
+
+You can only have one seed set loaded at a time because loading a seed set also
+sets your test up with an isolated database connection. This isolation allows
+us to automatically clean up the database between each test.
+
+### More Test Writing
+
+Check out the [Cypress docs](https://docs.cypress.io/) for much more information
+on writing tests with Cypress.
+
+## CI Setup
+
+We also run the Cypress suite in CI. We save the video of each test as
+an artifact so that you can download and view them.

apps/core/mix.exs

@@ -57,6 +57,7 @@ defmodule Legendary.Core.MixProject do
       "guides/features/background-jobs.md",
       "guides/features/content-management.md",
       "guides/features/devops-templates.md",
+      "guides/features/end-to-end-testing.md",
       "guides/features/email.md",
       "guides/features/feature-flags.md",
       "guides/features/i18n.md",