Skip tests with missing test vectors instead of failing.

This copes more gracefully when older version of the nist-kat package
are intalled that don't have newer test vectors such as CCM or plain
SHA.

If the nist-kat package is not installed at all, this still fails with
an error.

Reviewed by: cem
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D20034