ref-manual/classes.rst: document the testexport class

This class has been in OE-Core for a while but never documented in the
reference manual. Add some description for it and link to the existing
documentation on it.

(From yocto-docs rev: dd665216fa578a1f2f268790d708c6a5d2912ecf)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 362a331255525fc853dab3af4ec905c417fabb0b)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/classes.rst

@@ -3186,6 +3186,22 @@ class assuming :term:`PATCHRESOLVE` is set to "user", the
 :ref:`ref-classes-cml1` class, and the :ref:`ref-classes-devshell` class all
 use the :ref:`ref-classes-terminal` class.
 
+.. _ref-classes-testexport:
+
+``testexport``
+==============
+
+Based on the :ref:`ref-classes-testimage` class, the
+:ref:`ref-classes-testexport` class can be used to export the test environment
+outside of the :term:`OpenEmbedded Build System`. This will generate the
+directory structure to execute the runtime tests using the
+:oe_git:`runexported.py </openembedded-core/tree/meta/lib/oeqa/runexported.py>`
+Python script.
+
+For more details on how to use :ref:`ref-classes-testexport`, see
+the :ref:`test-manual/runtime-testing:Exporting Tests` section in the Yocto
+Project Test Environment Manual.
+
 .. _ref-classes-testimage:
 
 ``testimage``