classes/testexport: move to classes-recipe

This class should be in classes-recipe as, like testimage, it is specific
to image recipes.

This also solves a regression where simply IMAGE_CLASSES += "testexport"
doesn't work if testimage isn't already in IMAGE_CLASSES. The testexport
class adds testimage to IMAGE_CLASSES, but by this point it's too late.
Now that the class can only be inherited via IMAGE_CLASSES, we can
simply inherit testimage directly.

(From OE-Core rev: 87fcee9fed915ff3af528b976d7d44737f9f1a95)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/testexport.bbclass

@@ -7,12 +7,14 @@
 # To use it add testexport to global inherit and call your target image with -c testexport
 # You can try it out like this:
 # - First build an image. i.e. core-image-sato
-# - Add INHERIT += "testexport" in local.conf
+# - Add IMAGE_CLASSES += "testexport" in local.conf
 # - Then bitbake core-image-sato -c testexport. That will generate the directory structure
 #   to execute the runtime tests using runexported.py.
 #
 # For more information on TEST_SUITES check testimage class.
 
+inherit testimage
+
 TEST_LOG_DIR ?= "${WORKDIR}/testexport"
 TEST_EXPORT_DIR ?= "${TMPDIR}/testexport/${PN}"
 TEST_EXPORT_PACKAGED_DIR ?= "packages/packaged"
@@ -176,5 +178,3 @@ def testexport_create_tarball(d, tar_name, src_dir):
     tar.add(base_name)
     tar.close()
     os.chdir(current_dir)
-
-IMAGE_CLASSES += "testimage"