refkit_poky.py: split tests into more classes
refkit_poky.py: split tests into more classes

It is useful to run signature checks separately from the slower image
build tests. "bitbake -r refkit_poky" still runs both, as before, but
now refkit_poky.TestRefkitPokySignatures and
refkit_poky.TestRefkitPokyBuilds can be used to run those subsets of
the tests.

Signed-off-by: Patrick Ohly <0fc9d825c622b2a5ac5160289c29f03654b10648@intel.com>

jairglez/intel-iot-refkit,klihub/intel-iot-refkit,mythi/intel-iot-refkit,intel/intel-iot-refkit,klihub/intel-iot-refkit,jairglez/intel-iot-refkit,intel/intel-iot-refkit,YinThong/intel-iot-refkit,ipuustin/intel-iot-refkit,jairglez/intel-iot-refkit,ipuustin/intel-iot-refkit,jairglez/intel-iot-refkit,jairglez/intel-iot-refkit,mythi/intel-iot-refkit,klihub/intel-iot-refkit,jairglez/intel-iot-refkit,ipuustin/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,mythi/intel-iot-refkit,YinThong/intel-iot-refkit,YinThong/intel-iot-refkit,YinThong/intel-iot-refkit,ipuustin/intel-iot-refkit,ipuustin/intel-iot-refkit,YinThong/intel-iot-refkit,klihub/intel-iot-refkit,mythi/intel-iot-refkit,intel/intel-iot-refkit,intel/intel-iot-refkit,jairglez/intel-iot-refkit,intel/intel-iot-refkit,klihub/intel-iot-refkit,mythi/intel-iot-refkit,klihub/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,intel/intel-iot-refkit,intel/intel-iot-refkit,YinThong/intel-iot-refkit,YinThong/intel-iot-refkit,klihub/intel-iot-refkit