contributing: Add instructions on unit tests

UNIT_TESTS.md

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+# Unit Tests
+
+Besides the code of the module itself, it is important to submit in addition a
+suite of tests. They guarantee your add-on remains functional when changes are
+applied to the code on which it depends. Once committed to GitHub, the unit
+tests are run automatically by the CI/CD pipeline every time the code in the
+repository is modified. Notifications are issued if a modification somehow
+impacts your add-on.
+
+## Requirements
+
+The unit tests depend on software that you must install in your system
+beforehand: Subversion and the GRASS development package. On a packaged operating
+system like Ubuntu or Debian Linux this can be achieved with:
+
+```bash
+sudo apt install subversion grass-dev
+```
+
+If you have not done so yet, it is also helpful to install pre-commit. Follow
+[the
+instructions](https://github.com/OSGeo/grass/blob/main/doc/development/submitting/submitting.md#use-pre-commit)
+in the main GRASS repository.
+
+## Create a unit test
+
+Unit tests must be stored in a subfolder named `testsuite` inside the add-on
+folder. For example, if you contributed a raster module named r.mymodule you
+would run:
+
+```bash
+cd src/raster/r.mymodule
+mkdir testsuite
+```
+
+Then you can create a new Git branch and initiate a new unit test class:
+
+```bash
+git checkout new-branch
+vim testsuite/test_mymodule.py
+```
+
+The [unit testing
+documentation](https://grass.osgeo.org/grass-stable/manuals/libpython/gunittest_testing.html)
+in the GRASS Reference Manual provides an overview on how to create unit tests
+for C/C++ and Python code. Going through existing unit tests of a module
+like
+[r.sun](https://github.com/OSGeo/grass/blob/main/raster/r.sun/testsuite/test_rsun.py)
+is also insightful.
+
+## Run a unit test
+
+In most circumstances you will wish to run the unit tests against a local
+version of the add-on that you recently modified or created. To do so you need
+to install this local version on your GRASS installation, as in the example
+below.
+
+```bash
+grass /home/user/GRASSDATA/location/mapset
+g.extension r.mblend url=/home/user/git/grass-addons/src/raster/r.mymodule
+```
+
+You can then run the unit tests from the GRASS session, for example:
+
+```bash
+python3 testsuite/test_mymodule.py
+```
+
+## Submit a unit test
+
+The [general contribution
+guidelines](https://github.com/OSGeo/grass-addons/blob/master/CONTRIBUTING.md#changing-code-and-documentation)
+apply to unit tests. To facilitate the work of reviewers it is better to
+submit the unit tests in a dedicated pull request.