test(TPM::Attestation#key): update test setup

Now, when the algorithm is `ECDSA`, the `unique` field of the `pubArea`
should include two sized buffers instead of just one.

spec/tpm/key_attestation_spec.rb

@@ -173,7 +173,11 @@
         t_public.parameters.scheme = TPM::ALG_ECDSA
         t_public.parameters.curve_id = curve_id
         t_public.parameters.kdf = TPM::ALG_NULL
-        t_public.unique.buffer = attested_key.public_key.to_bn.to_s(2)[1..-1]
+
+        public_key_bytes = attested_key.public_key.to_bn.to_s(2)[1..-1]
+        coordinate_length = public_key_bytes.size / 2
+        t_public.unique.x.buffer = public_key_bytes[0..(coordinate_length - 1)]
+        t_public.unique.y.buffer = public_key_bytes[coordinate_length..-1]
 
         t_public.to_binary_s
       end