Test for malformed modulus in attestation cert

With this patch the attestation tests use the attested to key to sign a
message and use the public key in the attestation certificate to verify
the signature. Thereby tripping up over malformed public keys.

Bug: 118372436
Test: VtsHalKeymasterV4_0TargetTest
Change-Id: I4ce75c689cd5b6bb04a56e283c1202501ee821c9