doc: development-process: add notice on testing

Add testing notice to "Before creating patches" section.

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260123071523.1392729-1-dmantipov@yandex.ru>

Author: dmantipov

Documentation/process/5.Posting.rst

@@ -40,7 +40,12 @@ sending patches to the development community.  These include:
  - Test the code to the extent that you can.  Make use of the kernel's
    debugging tools, ensure that the kernel will build with all reasonable
    combinations of configuration options, use cross-compilers to build for
-   different architectures, etc.
+   different architectures, etc. Add tests, likely using an existing
+   testing framework like KUnit, and include them as a separate member
+   of your series (see the next section for more about patch series).
+   Note that this may be mandatory when affecting some subsystems. For
+   example, library functions (resides under lib/) are extensively used
+   almost everywhere and expected to be tested appropriately.
 
  - Make sure your code is compliant with the kernel coding style
    guidelines.