dev-manual: fix testimage usage instructions

The proper way to inherit the testimage class is now

IMAGE_CLASSES += "testimage"

so change lines with 'INHERIT += "testimage"' to match. This makes the
dev-manual consistent with the migration guide, which already specifies
that the old way causes an error.

(From yocto-docs rev: 4d660551a6547ffe216e53402759baef424c4da2)

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/dev-manual/runtime-testing.rst

@@ -229,7 +229,7 @@ The final thing you need to do when setting :term:`TEST_TARGET` to
    statements in your ``local.conf`` file::
 
       IMAGE_FSTYPES += "tar.gz"
-      INHERIT += "testimage"
+      IMAGE_CLASSES += "testimage"
       TEST_TARGET = "SystemdbootTarget"
       TEST_TARGET_IP = "192.168.2.3"
 
@@ -335,7 +335,7 @@ You can start the tests automatically or manually:
    inherit the :ref:`ref-classes-testimage` class by editing your
    ``local.conf`` file::
 
-      INHERIT += "testimage"
+      IMAGE_CLASSES += "testimage"
 
    Next, use BitBake to run the tests::
 

documentation/dev-manual/upgrading-recipes.rst

@@ -117,7 +117,7 @@ The following steps describe how to set up the AUH utility:
       class, which is optional, you need to have the following set in
       your ``conf/local.conf`` file::
 
-         INHERIT += "testimage"
+         IMAGE_CLASSES += "testimage"
 
       .. note::