Provide hook for tests to know server's SPN and UPN

[roncain]

Right now, SPN and UPN tests require user to manually provide the NegotiateTestSPN and NegotiateTestUPN. If possible, we need a test hook so the test can ask the server for its SPN and UPN so this can be automated.

If this is possible, the ConditionalFact detector for SPN_Available and UPN_Available can use them and allow the dependent tests to light up (whereas today they are purely manual)