Add initial test coverage for RFC 3779 code.

Add initial test coverage for RFC 3779 code.

This exercises the code paths that are reached from the validator
and also tests that the public API behaves as expected. There is a
lot more that could be done here, but this test is already big enough.

Missing are tests for X509v3_{addr,asid}_validate_{path,resource_set}()
themselves.

One test failure is ignored and will be fixed in the near future
when a bad logic error in range_should_be_prefix() is fixed.
A consequence of this bug is that we will currently accept and generate
DER that doesn't conform to RFC 3779.