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.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>