Fix vehicle manifest test in test_primary
Fix vehicle manifest test in test_primary

This was broken by switch to using ASN.1/DER as default format.
We tell tuf.keys.verify_signature that in this test the data we're
passing in to verify is not DER, but a basic Python dictionary,
despite whatever tuf.conf.METADATA_FORMAT is set to.

awwad/uptane,uptane/uptane,awwad/uptane,shikhar394/uptane,uptane/uptane,shikhar394/uptane