docs(contr): add section on testing

Author: jsjoeio

CHANGELOG.md

@@ -60,6 +60,7 @@ VS Code v0.00.0
 - chore: update node to v14 #3458 @oxy
 - chore: update .gitignore #3557 @cuining
 - fix: use sufficient computational effort for password hash #3422 @jsjoeio
+- docs(CONTRIBUTING): add section on testing #3629 @jsjoeio
 
 ### Development
 

docs/CONTRIBUTING.md

@@ -1,5 +1,6 @@
 <!-- START doctoc generated TOC please keep comment here to allow auto update -->
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
 # Contributing
 
 - [Pull Requests](#pull-requests)
@@ -9,6 +10,10 @@
   - [Updating VS Code](#updating-vs-code)
     - [Notes about Changes](#notes-about-changes)
 - [Build](#build)
+- [Testing](#testing)
+  - [Unit Tests](#unit-tests)
+  - [Integration Tests](#integration-tests)
+  - [End-to-End Tests](#end-to-end-tests)
 - [Structure](#structure)
   - [Modifications to VS Code](#modifications-to-vs-code)
   - [Currently Known Issues](#currently-known-issues)
@@ -112,6 +117,43 @@ In our GitHub Actions CI, we use CentOS 7 for maximum compatibility.
 If you need your builds to support older distros, run the build commands
 inside a Docker container with all the build requirements installed.
 
+## Testing
+
+There are three kinds of tests in code-server:
+
+1. unit tests
+2. integration tests
+3. end-to-end tests
+
+### Unit Tests
+
+Our unit tests are written in TypeScript and run using [Jest](https://jestjs.io/), which is a JavaScript testing framework.
+
+These live under [test/unit](../test/unit).
+
+We use unit tests for functions and things that can be tested in isolation.
+
+### Integration Tests
+
+These are a work-in-progress. We build code-server and run a script called [test-standalone-release.sh`](../ci/build/test-standalone-release.sh)
+which ensures that code-server's CLI is working.
+
+Integration for us means testing things that integrate and rely on each other. For instance, testing the CLI which requires that code-server be built and packaged.
+
+### End-to-End Tests
+
+The end-to-end (e2e) are written in TypeScript and run using [Playwright](https://playwright.dev/).
+
+These live under [test/e2e](../test/e2e).
+
+Before the e2e tests run, we have a `globalSetup` that runs which makes it so you don't have to login before each test and can reuse the authentication state.
+
+Take a look at `codeServer.test.ts` to see how you use it (look at `test.use`).
+
+We also have a model where you can create helpers to use within tests. Take a look at [models/CodeServer.ts](../test/e2e/models/CodeServer.ts) to see an example.
+
+Generally speaking, e2e means testing code-server running in the browser, similar to how a user would interact with it. When running these tests with `yarn test:e2e`, you must have code-server running locally. In CI, this is taken care of for you.
+
 ## Structure
 
 The `code-server` script serves an HTTP API for login and starting a remote VS Code process.