docs/develop: Add MicroPython Internals chapter.

This commit adds many new sections to the existing "Developing and building
MicroPython" chapter to make it all about the internals of MicroPython.

This work was done as part of Google's Season of Docs 2020.

1 files changed, 70 insertions, 0 deletions

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+.. _writingtests:
+
+Writing tests
+=============
+
+Tests in MicroPython are located at the path ``tests/``. The following is a listing of
+key directories and the run-tests runner script:
+
+.. code-block:: bash
+
+   .
+    ├── basics
+    ├── extmod
+    ├── float
+    ├── micropython
+    ├── run-tests
+    ...
+
+There are subfolders maintained to categorize the tests. Add a test by creating a new file in one of the
+existing folders or in a new folder. It's also possible to make custom tests outside this tests folder,
+which would be recommended for a custom port.
+
+For example, add the following code in a file ``print.py`` in the ``tests/unix/`` subdirectory:
+
+.. code-block:: python
+
+   def print_one():
+       print(1)
+
+   print_one()
+
+If you run your tests, this test should appear in the test output:
+
+.. code-block:: bash
+
+   $ cd ports/unix
+   $ make tests
+   skip  unix/extra_coverage.py
+   pass  unix/ffi_callback.py
+   pass  unix/ffi_float.py
+   pass  unix/ffi_float2.py
+   pass  unix/print.py
+   pass  unix/time.py
+   pass  unix/time2.py
+
+Tests are run by comparing the output from the test target against the output from CPython.
+So any test should use print statements to indicate test results.
+
+For tests that can't be compared to CPython (i.e. micropython-specific functionality),
+you can provide a ``.py.exp`` file which will be used as the truth for comparison.
+
+The other way to run tests, which is useful when running on targets other than the Unix port, is:
+
+.. code-block:: bash
+
+   $ cd tests
+   $ ./run-tests
+
+Then to run on a board:
+
+.. code-block:: bash
+
+   $ ./run-tests --target minimal --device /dev/ttyACM0
+
+And to run only a certain set of tests (eg a directory):
+
+.. code-block:: bash
+
+   $ ./run-tests -d basics
+   $ ./run-tests float/builtin*.py