s4:drs:test:getncchanges skips some tests with reserved_usn = 0

s4:drs:test:getncchanges skips some tests with reserved_usn = 0

These tests are not affected by the reserved_usn change, so there is
no need to run them twice.

The test_repl_get_tgt_multivalued_links fails with or without
reserved_usn set to zero, but it fails differently in either case.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15701

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jennifer Sutton <josutton@catalyst.net.nz>