imagefeatures: New test case, test_empty_image, added

imagefeatures: New test case, test_empty_image, added

An empty image build file exists under the meta-selftest folder, test-empty-image.bb,
which builds an image with no additional packages. However, there were no further
selftest created to verify its emptiness

This change consists of the selftest related to the said image to verify its
emptiness and the 'import glob' moved as global import instead of local import.

The expected outcome of the test should be TRUE or 1 if the .manifest file content
is empty.

[YOCTO #8455]

Signed-off-by: Khairul Rohaizzat Jamaluddin <khairul.rohaizzat.jamaluddin@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>