Improve test descriptions

[jku]

From #218 :

test/assets/bundle-verify/bundle-with-root-cert_fail/README:

bundle certificate chain contains a root certificate: client should reject this

why should client reject this?

IIRC the client spec says this but not 100% sure.

test/assets/bundle-verify/intoto-expired-certificate_fail/README:

Certificate was issued outside the validity window defined in trusted root

Is this for an expired CA certificate? If so rename this case to intoto-expired-ca-certificate_fail

To me the description sounds like the issue is only the validity window defined in the trusted root file for fulcio but I don't know for sure.

test/assets/bundle-verify/dsse-invalid-sig_fail/README:

The title here is self-explanatory, but this should still have some description on the reason for failure. For example, if the signature has the incorrect length.