docs: Add Testing Guidelines

This section describes the testing framework and format standards for
tests in crostests package.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

docs/source/development/testguide.rst

 ******************
 Testing Guidelines
 ******************
+
+This section describes the testing framework and format standards for tests in
+``crostests`` packages.
+
+Testing Framework
+*****************
+
+The testing framework used by ``crostests``  is the `unittest`_ framework.
+
+.. _unittest: https://docs.python.org/3/library/unittest.html
+
+Running Tests
+*************
+
+The first thing you should do is download latest source version::
+
+    git clone https://gitlab.collabora.com/chromiumos/crostests.git
+    cd crostests
+
+At the root of the project, add the current directory in the PYTHONPATH::
+
+    export PYTHONPATH=${PYTHONPATH}:${PWD}
+
+There are currently different ways to invoke ``crostests``. Each method
+invokes `unittest`_ to run the tests but offers different options when
+calling. To run the tests, you will need to make sure you have the `unittest`_
+package is installed.
+
+Using a runner
+==============
+
+This is the simplest way to run all the tests, just start a runner with::
+
+    python3 -m cros.runners.lava_runner
+
+Using the unittest Command-Line Interface
+=========================================
+
+The unittest module can be used from the command line to run tests from
+modules::
+
+    python -m unittest cros.tests.cros_ec_rtc
+
+You can run tests with more detail (higher verbosity) by passing in the -v flag::
+
+    python -m unittest -v cros.tests.cros_ec_rtc
+
+Writing tests
+*************
+
+Simple example
+==============
+
+The following example shows a simple function and a test to test this
+function::
+
+    #!/usr/bin/env python3
+    # -*- coding: utf-8 -*-
+
+    import unittest
+
+    def func(x):
+        """Add one to the argument."""
+        return x + 1
+
+    class TestSimple(unittest.TestCase):
+        def test_answer(self):
+            """Check the return value of func() for an example argument."""
+            self.assertEqual(func(3), 5)
+
+If we place this in ``cros/tests/example.py`` file and then run::
+
+    python3 -m unittest cros.tests.example
+
+The result is::
+
+    F
+    ======================================================================
+    FAIL: test_answer (cros.tests.example.TestSimple)
+    Check the return value of func() for an example argument.
+    ----------------------------------------------------------------------
+    Traceback (most recent call last):
+      File "~/crostests/cros/tests/example.py", line 13, in test_answer
+          self.assertEqual(func(3), 5)
+    AssertionError: 4 != 5
+
+    ----------------------------------------------------------------------
+    Ran 1 test in 0.000s
+
+    FAILED (failures=1)
+
+Where to put tests
+==================
+
+Package-specific tests
+----------------------
+
+Each package should include a suite of unit tests, covering as many of
+the public methods/functions as possible. These tests should be
+included inside each sub-package, e.g::
+
+    cros/tests/
+
+``tests`` directories should contain an ``__init__.py`` file so that
+the tests can be imported and so that they can use relative imports.
+
+Regression tests
+================
+
+One of the main focus of this framework is catch kernel regressions on
+future releases using KernelCI. Any time a kernel bug is fixed, and
+wherever possible, one or more regression tests should be added to ensure
+that the bug is not introduced in future. Regression tests should include
+a description of the reported bug.
+