functionaliy for conducting tests based upon a canned, preconfigured v6 subnet.
Since the new tests needed to be performed against an "empty" configuration,
I split out the common functionaliy into a base class text fixture, NakedDhcp6SvrTest,
and altered Dhcp6SvrTest to derive from it.   The new base class has no
preconfiguation.  What remains in Dhcp6SvrTest is geared soley towards tests against
a valid, canned, configuration.

The new test_fixture NakedDhcp6SrvTest is used for the following new tests:

    SolicitNoSubnet - checks response to a SOLICT w/o subnet defined
    RequestNoSubnet - checks response to a REQUEST w/o subnet defined
    RenewNoSubnet - checks response to a RENEW w/o subnet defined
    ReleaseNoSubnet - checks response to a RELEASE w/o subnet defined