[3352] Augment DHPC6 FQDN tests to properly check handling of FQDN flags

Unit tests did not detect that Dhcpv6Srv::createNameChangeRequests attempts
to make NCRs when the response FQDN flags indicate that none should be created.
FqdnDhcpv6Srv::testFqdn has been expanded to invoke this method and check the
outcome.
[3352] Added test to not create NCRs if FQDN flags indicate none required

Dhcpv6Srv::createNameChangeRequests was not catching cases where FQDN flags
indicate NO changes should be made. This causes an exception to thrown by
the NCR constructor stating that both forward and reverse flags cannot be
false. Also moved the check up in the method, above of the options loop.
[3352] Addressed review comments.

Added use of boolean structs for fdqn unit tests for clarity.
Minor cosmetics.