Update the mbedtls_test_psa_exercise_key to handle and use
PSA_KEY_USAGE_SIGN_MESSAGE and PSA_KEY_USAGE_VERIFY_MESSAGE key policies.
Add new tests for PSA_KEY_USAGE_SIGN_MESSAGE and PSA_KEY_USAGE_VERIFY_MESSAGE
policies.
Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>