selftest: Do not test for link ordering in tombstones_expunge test

selftest: Do not test for link ordering in tombstones_expunge test

By testing only for the DNs that are returned we do not change the strictness of
the test, because it is a test of the match rule which applies to the whole
object, not the returned values.

However, when this code asserted the returned order of the links, it prevents
us from changing this order.  This order was not deterministic across DCs
but as this test ran against an offline DB, it was able to assume a
particular order.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>