doc: Document how sandbox_spl_tests are run

Add a few notes about the sandbox_spl tests, since they are special.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Pratyush Yadav <p.yadav@ti.com>

doc/develop/testing.rst

@@ -58,6 +58,11 @@ also select particular tests with -k::
 
    ./test/py/test.py --bd sandbox_flattree --build -k hello
 
+There are some special tests that run in SPL. For this you need the sandbox_spl
+build::
+
+   ./test/py/test.py --bd sandbox_spl --build -k test_spl
+
 See test/py/README.md for more information about the pytest suite.
 
 See :doc:`tests_sandbox` for how to run tests directly (not through pytest).