[#13,!6] - Added unit tests to bin/dhcp6 to verify global HRs

[#13,!6] - Added unit tests to bin/dhcp6 to verify global HRs

src/bin/dhcp6/tests/dhcp6_test_utils.cc
    Minor mods to emit configuration/parsing errors

src/bin/dhcp6/tests/host_unittest.cc
    HostTest::sarrTest() - new function for testing a
    SARR that should result in a lease

    TEST_F(HostTest, globalReservationsNA)
    TEST_F(HostTest, globalReservationsPD) - new tests
    to verify global HRs