tests/getnc_exop: Improve the ridalloc test by performing an alloc against a new...

tests/getnc_exop: Improve the ridalloc test by performing an alloc against a new master

Currently we fail against ourselves due to rIDNextRid and
rIDPreviousAllocationPool normally being unset, despite being mandatory
attributes (being the only attributes in this situation).

Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Pair-programmed-with: Clive Ferreira <cliveferreira@catalyst.net.nz>
Signed-off-by: Bob Campbell <bobcampbell@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=12394