Make alternative named builds testable in system tests
    
    Red Hat has alternative variant builds of named, which are not ever
    tested by system tests. New variables make it relatively easy to test
    alternative variants.
    
    For sdb variant use:
    export NAMED_VARIANT=-sdb DNSSEC_VARIANT=
    
    For pkcs variant use:
    export NAMED_VARIANT=-pkcs11 DNSSEC_VARIANT=-pkcs11
    
    followed by make test in build directory.
    
    Note: PKCS11 tests are still skipped, it requires SLOT variable
    exported. Fails in some cases.