Attest: Enhance test suite to parse and validate token

Integrate attestation test suite with CBOR/COSE parser:
 - Parse the token and verify the presence and the value
   of the claims
 - Verify the signature of the token

Change-Id: I169eb0ba96eb81ff759328a5628a5147545ccd13
Signed-off-by: Tamas Ban <tamas.ban@arm.com>