docs: for Cypress testing

docs/07-testing.md

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+# Cypress testing
+
+Cypress tests come in two types: mocked and end-to-end (E2E).
+
+## Mocked tests
+
+Most of the tests are mocked tests. They mock responses from various APIs and
+short-circuit the identity middleware, which prevents the tests from needing to
+log a user in. The middleware is short-circuited by setting the value of the
+global variable `CYPRESS` to `'SKIP_IDAPI'` in the `bundle-dev-server-cypress`
+script in `package.json`.
+
+The mocked tests live in `cypress/tests/mocked`.
+
+The mocked tests use the following commands in `package.json`:
+
+-   `yarn cypress:mocked:server`: starts a dev server (essentially the same as the standard `yarn watch` command but with the `CYPRESS` variable set to `'SKIP_IDAPI'`, and doesn't open a new browser tab).
+-   `yarn cypress:mocked:open`: opens the Cypress test runner in mocked mode.
+-   `yarn cypress:mocked:run`: runs the Cypress tests headless in mocked mode.
+
+In CI, the mocked tests are run by the `cypress-mocked.yml` GitHub Actions job.
+
+## End-to-end tests
+
+The end-to-end tests live in `cypress/tests/e2e`. They currently comprise a
+small suite of basic tests which use CODE IDAPI to generate a test user and CODE
+Gateway to sign that user in before performing a few basic functions on the
+frontend.
+
+To run E2E tests locally, it is necessary to download a Cypress env file from S3 with the following command (after getting fresh
+tokens from Janus):
+
+```bash
+aws s3 cp --profile membership s3://gu-reader-revenue-private/manage-frontend/CODE/cypress.env.json cypress.env.json
+```
+
+Also ensure that you have run the setup script in the `nginx` directory, which
+sets up the local Nginx proxy to CODE Gateway and IDAPI.
+
+The E2E tests use the following commands in `package.json`:
+
+-   `yarn cypress:e2e:server`: starts a dev server without setting the `CYPRESS` variable.
+-   `yarn cypress:e2e:open`: opens the Cypress test runner in E2E mode.
+-   `yarn cypress:e2e:run`: runs the Cypress tests headless in E2E mode.