tests: Fix flappiness in DRS tests due to RID Set changing

tests: Fix flappiness in DRS tests due to RID Set changing

The test_link_utdv_hwm test case in getnc_exop has started getting
slightly flappy (8 failures in the last 2 weeks). The problem is the
test case creates a new computer, which can occasionally result in a new
RID pool being allocated.

The problem can be reproduced by running the test case repeatedly (it
usually fails after ~250 times).

This patch updates the _check_ctr6() assertion to filter out the 'CN=RID
Set' object, if it happens to be present.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>