documentation: unit tests: extend description

Add a more detailed description on how to unit test using ztest and twister


Signed-off-by: Stefan Schmidt <[email protected]>

Author: stemschmidt

doc/develop/test/ztest.rst

@@ -360,7 +360,16 @@ efforts into the specific module in question. This will speed up testing since
 only the module will have to be compiled in, and the tested functions will be
 called directly.
 
-Examples of unit tests can be found in the :zephyr_file:`tests/unit/` folder.
+To setup unit tests you have to add a CMakeLists.txt, a testcases.yml and a
+prj.conf to the directory containing the unit test source files. The resulting
+binary from this directory is build with the -DBOARD=unit_testing. When twister
+is invoked the script zephyr/scripts/pylib/twister/twisterlib/testplan.py
+filters out all testcases.yml in which type: unit is not set. Only unit tests
+are executed with a firmware build with BOARD=unit_testing.
+
+CMakeLists.txt
+==============
+
 In order to declare the unit tests present in a source folder, you need to add
 the relevant source files to the ``testbinary`` target from the CMake
 :zephyr_file:`unittest <cmake/modules/unittest.cmake>` component. See a minimal
@@ -383,6 +392,41 @@ and are used to decide whether a test failed or passed by verifying whether an
 interaction with an object occurred, and if required, to assert the order of
 that interaction.
 
+testcases.yaml
+==============
+
+You have to set the value for the key "type" to "unit" in the testcase.yaml
+
+.. code-block:: yaml
+
+   tests:
+      testscenario.testsuite:
+         tags: your_tag
+         type: unit
+
+prj.conf
+========
+
+For unit tests this contains usually only
+
+.. code-block:: kconfig
+
+   CONFIG_ZTEST=y
+
+If your unit tests require additional libraries (e.g. math-lib) you will have to
+add them either via the CMakeLists.txt or in the testcase.yaml:
+
+.. code-block:: yaml
+
+   tests:
+      testscenario.testsuite:
+         tags: your_tag
+         type: unit
+         extra_args:
+            - EXTRA_LDFLAGS="-lm"
+
+Examples of unit tests can be found in the :zephyr_file:`tests/unit/` folder.
+
 Best practices for declaring the test suite
 *******************************************