docs/devel: add forward reference to check-tcg

For completeness reference the check-tcg tests in the container
preamble text.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Message-Id: <20210222101455.12640-7-alex.bennee@linaro.org>

docs/devel/testing.rst

@@ -289,6 +289,9 @@ an alternative container runtime. Although the many of the target
 names and scripts are prefixed with "docker" the system will
 automatically run on whichever is configured.
 
+The container images are also used to augment the generation of tests
+for testing TCG. See :ref:`checktcg-ref` for more details.
+
 Docker Prerequisites
 --------------------
 
@@ -1011,6 +1014,8 @@ And remove any package you want with::
 If you've used ``make check-acceptance``, the Python virtual environment where
 Avocado is installed will be cleaned up as part of ``make check-clean``.
 
+.. _checktcg-ref:
+
 Testing with "make check-tcg"
 =============================