doc: Explain briefly how to write new tests

Add a second on writing tests, covering when to use Python and C, where
to put the tests, etc. Add a link to the existing Python test
documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/testing.rst

@@ -117,6 +117,8 @@ or is covered sparingly. So here are some suggestions:
   is much easier to add onto a test - writing a new large test can seem
   daunting to most contributors.
 
+See doc:`tests_writing` for how to write tests.
+
 
 Future work
 -----------