Add "Missing Security Level" Test

It is possible for the key security level to be omitted from the
key container. When this happens, SW_SECURE_CRYPTO should be used
as the key's security level (as per the protobuf definition).

This only matters when reading the security level from the key
container since the security level must appear in the key control
block.

This change adds a test that will purposely omit the key security
level from the key container.

whitebox/api/BUILD

@@ -138,6 +138,7 @@ cc_library(
         "license_whitebox_masked_decrypt_test.cc",
         "license_whitebox_process_license_response_core_message_test.cc",
         "license_whitebox_process_license_response_test.cc",
+        "license_whitebox_security_level_test.cc",
         "license_whitebox_sign_license_request_test.cc",
         "license_whitebox_sign_renewal_request_test.cc",
         "license_whitebox_test_base.cc",