doc: add How to write a Node.js test guide

CONTRIBUTING.md

@@ -133,8 +133,9 @@ $ git rebase upstream/master
 ### Step 5: Test
 
 Bug fixes and features **should come with tests**. Add your tests in the
-test/parallel/ directory. Look at other tests to see how they should be
-structured (license boilerplate, common includes, etc.).
+`test/parallel/` directory. For guidance on how to write a test for the Node.js
+project, see this [guide](./doc/guides/writing_tests.md). Looking at other tests
+to see how they should be structured can also help.
 
 ```text
 $ ./configure && make -j8 test

doc/guides/writing_tests.md

@@ -0,0 +1,186 @@
+# How to write a test for the Node.js project
+
+## What is a test?
+
+A test must be a node script that exercises a specific functionality provided
+by node and checks that it behaves as expected. It should return 0 on success,
+otherwise it will fail. A test will fail if:
+
+- It exits by calling `process.exit(code)` where `code != 0`
+- It exits due to an uncaught exception.
+- It never exits. In this case, the test runner will terminate the test because
+  it sets a maximum time limit.
+
+Tests can be added for multiple reasons:
+
+- When adding new functionality.
+- When fixing regressions and bugs.
+- When expanding test coverage.
+
+
+## Test structure
+
+Let's analyze this very basic test from the Node.js test suite:
+
+```javascript
+1  'use strict'; 
+2  const common = require('../common');
+3  const http = require('http');
+4  const assert = require('assert');
+5
+6  const server = http.createServer(common.mustCall((req, res) => {
+7    res.end('ok');
+8  }));
+9  server.listen(common.PORT, () => {
+10   http.get({
+11     port: common.PORT,
+12     headers: {'Test': 'Düsseldorf'}
+13   }, common.mustCall((res) => {
+14     assert.equal(res.statusCode, 200);
+15     server.close();
+16   }));
+17 });
+```
+
+**Lines 1-2**
+
+```javascript
+'use strict';
+const common = require('../common');
+```
+
+These two lines are mandatory and should be included on every test.
+The `common` module is a helper module that provides useful tools for the tests.
+If for some reason, no functionality from `common` is used, it should still be
+included like this:
+
+```javascript
+require('../common');
+```
+
+Why? It checks for leaks of globals.
+
+**Lines 3-4**
+
+```javascript
+const http = require('http');
+const assert = require('assert');
+```
+
+These modules are required for the test to run. Except for special cases, these
+modules should only include core modules.
+The `assert` module is used by most of the tests to check that the assumptions
+for the test are met.
+
+**Lines 6-17**
+
+This is the body of the test. This test is quite simple, it just tests that an
+HTTP server accepts `non-ASCII` characters in the headers of an incoming
+request. Interesting things to notice:
+
+- The use of `common.PORT` as the listening port. Always use `common.PORT`
+  instead of using an arbitrary value, as it allows to run tests in parallel
+  safely, as they are not trying to reuse the same port another test is already
+  using.
+- The use of `common.mustCall` to check that some callbacks/listeners are
+  called.
+- The HTTP server is closed once all the checks have run. This way, the test can
+  exit gracefully. Remember that for a test to succeed, it must exit with a
+  status code of 0.
+
+## General recommendations
+
+### Timers
+
+The use of timers is discouraged, unless timers are being tested. There are
+multiple reasons for this. Mainly, they are a source of flakiness. For a thorough
+explanation go [here](https://github.com/nodejs/testing/issues/27).
+
+In the event a timer is needed, it's recommended using the
+`common.platformTimeout()` method, that allows setting specific timeouts
+depending on the platform. For example:
+
+```javascript
+const timer = setTimeout(fail, common.platformTimeout(4000));
+```
+
+will create a 4-seconds timeout, except for some platforms where the delay will
+be multiplied for some factor.
+
+### The *common* API
+
+Make use of the helpers from the `common` module as much as possible.
+
+One interesting case is `common.mustCall`. The use of `common.mustCall` may
+avoid the use of extra variables and the corresponding assertions. Let's explain
+this with a real test from the test suite.
+
+```javascript
+'use strict';
+var common = require('../common');
+var assert = require('assert');
+var http = require('http');
+
+var request = 0;
+var response = 0;
+process.on('exit', function() {
+  assert.equal(request, 1, 'http server "request" callback was not called');
+  assert.equal(response, 1, 'http request "response" callback was not called');
+});
+
+var server = http.createServer(function(req, res) {
+  request++;
+  res.end();
+}).listen(common.PORT, function() {
+  var options = {
+    agent: null,
+    port: this.address().port
+  };
+  http.get(options, function(res) {
+    response++;
+    res.resume();
+    server.close();
+  });
+});
+```
+
+This test could be greatly simplified by using `common.mustCall` like this:
+
+```javascript
+'use strict';
+var common = require('../common');
+var assert = require('assert');
+var http = require('http');
+
+var server = http.createServer(common.mustCall(function(req, res) {
+  res.end();
+})).listen(common.PORT, function() {
+  var options = {
+    agent: null,
+    port: this.address().port
+  };
+  http.get(options, common.mustCall(function(res) {
+    res.resume();
+    server.close();
+  }));
+});
+
+```
+
+### Flags
+
+Some tests will require running Node.js with specific command line flags set. To
+accomplish this, a `// Flags: ` comment should be added in the preamble of the
+test followed by the flags. For example, to allow a test to require some of the
+`internal/*` modules, the `--expose-internals` flag should be added.
+A test that would require `internal/freelist` could start like this:
+
+```javascript
+'use strict';
+
+// Flags: --expose-internals
+
+require('../common');
+const assert = require('assert');
+const freelist = require('internal/freelist');
+```