classes/testimage-auto: add class to allow automatically running image tests
Setting TEST_IMAGE = "1" alone will now automatically run tests on the
image immediately after the image is built instead of having to add
INHERIT += "testimage" and run bitbake -c testimage <image> manually
(but that will still work). This restores functionality that was
present in the older imagetest-qemu class with IMAGETEST.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
