This commit adds "generate_test_keys.py" script to generate
predefined keys used in test_suite_pk. Keys are generated with
"programs/pkey/gen_key" tool and converted to C array using
the python script.
tests/src/test_keys.h is automatically generated using the
above mentioned script.
test_suite_pk is updated in order to use the new format.
Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>